RAID & Disk Array Recovery

What is RAID?

RAID – Redundant Array of Intelligent/Inexpensive Disks.

RAID is a great system for increasing speed and availability of data as it offers considerably more data protection than non-RAID disk systems. However, its management of the disks and the data distribution across them can be extremely complex.

Vogon has extensive experience of recovery from RAID, spanned and striped systems. We only normally require just the storage devices in order to recover lost data. If you are asked for the original RAID or array system by a Data Recovery company, you must ask yourself whether that company really understands how the system works.

Different RAID levels are utilised for different applications, depending upon the fault tolerance, speed of access required or the average size of the files being stored.

RAID systems do fail!
Not something that a RAID vendor would want to admit to, but complex redundant systems can suffer from failure. Often not a fault of the technology used or the design of the array, but typically failure to correctly implement these systems leads to a single point of failure that can cause catastrophic data loss.

No matter how well designed or implemented the system is, there is still one very complex factor that causes most RAID array problems we see – the human being. Mistakes are easy to make and the more complex the systems become, the more potential there is for mistakes to occur.

Vogon has an excellent working relationship with all of the major peripheral, software and media manufacturers and has developed custom software and firmware for major American and Japanese peripheral manufacturers and software companies.


Vogon RAID and array recovery processes
Using our disk recovery processes, coupled with our ability to produce a safe 'copy' of the complete volume, allows us to process an array as a collection of image files. Our Data Recovery systems have the capacity to absorb most server array volumes.

A project team of Data Recovery Hardware Specialists and Data Recovery Software Specialists is formed to deal with each recovery. While the failed drives are recovered onto the Data Recovery servers by the hardware team, the software team evaluate the RAID or array images, work out the configuration and determine the extent of the corruption that typically occurs when these systems fail in operation.

The advanced software tools we have written will extract the data from the images. When a drive image is not available, the tools can reconstruct the data 'on-the-fly' in the same way that the RAID rebuild process would have done on the original system.

At all stages, controlling the tools is an experienced software expert who closely understands the software tools, the array configuration, the file system on the array, what the problem is and why the array failed in the first place.

The data will be returned using the customer's chosen media.

What are the RAID levels?

RAID 0 – Striped drives without parity, a non-redundant group


This is the fastest and most storage efficient configuration of a group of disks, but it has no fault tolerance; if one drive fails the whole array fails. As there is no parity, it offers the best storage efficiency. The striping allows read/write operations to occur simultaneously on each disk for speed.


RAID 1 – Mirroring drives (vedi anche:
Tecnologia RAID: mirroring, parità e stripping)


The fastest fault tolerant array configuration but the least storage efficient array, it's probably the most commonly used system today. As other array configurations require three or more drives, it is the only choice in a two-drive system. The two drives in the 'mirrored pair' appear to the operating system as one; they always mirror each other. Should a drive fail the data is available from the other. As the data is always the same on the two drives, read performance is almost doubled, as consecutive reads can occur on each drive at the same time. Write performance is not improved.

RAID 2 – Sector striping with a disk holding Error Correction Code (ECC) information

As all hard disks have ECC data built into every sector, this configuration has no advantages over RAID 3 and is not used.

RAID 3 – Sector striping with a disk holding parity information


A single disk holds the parity information and the data integrity checking relies on ECC on the disk drives themselves, as with RAID 2. Because parity information is held on a dedicated drive, any single drive in the array can fail without data being lost. Data can be rebuilt onto a replacement drive using the remaining information and the parity information. To maintain performance when transferring small files, synchronised motors may be required on each disk. Read and write operations cannot be overlapped, but as consecutive reads can occur on each drive at the same time, read performance is improved.


RAID 4 – Very large sector striping with a disk holding parity information

This is a faster version of RAID 3, in that the complete read of a file can occur on a single disk. Write performance is not improved as the parity drive needs updating with every write operation. As there are no real benefits over RAID 3 and write performance is worse than RAID 5, this configuration is not used.

RAID 5 – Large sector striping data with a rotating parity stripe


With no dedicated parity drive the write performance is better than RAID 3, with overlapped data and parity update writes. Read operations can be interleaved as all drives contain data stripes. RAID 1 performs faster but RAID 5 provides better storage efficiency. Parity update can be more efficiently handled by RAID 5, by checking for data bit changes and only changing the corresponding parity bits. For small data writes improvements here are lost as most disk drives update sectors entirely for any write operation. For larger writes only the sectors where bit changes need to be made are rewritten and improvements made. Maintaining parity information reduces write performance – in some cases as low as one third the speed of RAID 1. For this reason RAID 5 is not normally used in performance critical processes.

Striping
RAID accomplishes the formation of a single logical drive from multiple units through a process called striping. Striping involves logically arranging information so that individual files are spread among a group of drives. A stripe segment can be as small as a single byte, or as large as multiple sectors. Striping has the advantage of faster data access because individual drives can be accessed in parallel. The disadvantage is that the array capacity, once formatted, is fixed, meaning expansion is not as simple as adding drive modules.

RAID Levels Compared

RAID level

AKA

Minimum drives required

Advantages

Disadvantages

Typical applications

0

Striped array.

Data is stored across multiple drives.

2

Very high read and write performance—almost that of a single drive multiplied by the number of drives used. Technically not a RAID but an “AID” because it has no redundancy.

The failure of any drive compromises the entire array.

Environments where raw speed is valued over data integrity. Video editing stations, high-end game servers, etc.

1

Mirrored array.

Data is duplicated on separate drives

2

Better read performance than a single drive.

Full redundancy protects data from the failure of any one drive in the array.

Capacity is half the total storage available on member drives.

Write performance is marginally worse than a single drive.

Environments where data integrity is favoured over raw speed. Web servers, database servers, etc.

5

Striped array with distributed parity.

Data and parity information are stored across multiple drives.

3

Superior performance compared with RAID 1.

Better storage efficiency compared with RAID 1.

Parity information provides data protection though error checking and correction.

Not as fast a RAID 0.

Parity calculations and storage reduces array performance and capacity, respectively.

Requires expensive controller card and more drives to implement.

Environments where data integrity is paramount, but speed is also highly desirable, such as enterprise servers.
0+1 Mirrored array of striped sets of drives.

Data is stored and duplicated across multiple drives.

4

RAID 0 performance with RAID 1 redundancy.

Superior performance to RAID 5 with data security.

 

Requires a minimum of four drives and expensive controller card. Not as robust as RAID 1+0.

 

Environments seeking exceptional performance as well as data integrity.
1+0 Striped array of mirrored pairs of drives.

Data is stored and duplicated across multiple drives.

4

RAID 0 performance with RAID 1 redundancy.

Superior performance to RAID 5 with data security.

Requires a minimum of four drives and expensive controller card. More robust than RAID 0+1. Environments seeking exceptional performance as well as data integrity.

 

RAID Level

Number of Disks

Capacity

Storage Efficiency

Fault Tolerance

Availability

Random Read Perf

Random Write Perf

Sequential Read Perf

Sequential
Write Perf

Cost

0

2,3,4,... S*N 100% none 1.0 stars 4.0 stars 4.0 stars 4.5 stars 4.0 stars $

1

2 S*N/2 50% 4.0 stars 4.0 stars 3.0 stars 3.0 stars 2.0 stars 3.0 stars $$

2

many varies, large ~ 70-80% 2.0 stars 4.0 stars 2.0 stars 1.0 stars 4.0 stars 2.5 stars $$$$$

3

3,4,5,... S*(N-1) (N-1)/N 3.0 stars 4.0 stars 3.0 stars 1.0 stars 4.0 stars 2.5 stars $$

4

3,4,5,... S*(N-1) (N-1)/N 3.0 stars 4.0 stars 4.0 stars 1.5 stars 3.0 stars 2.0 stars $$

5

3,4,5,... S*(N-1) (N-1)/N 3.0 stars 4.0 stars 4.5 stars 2.0 stars 3.5 stars 2.5 stars $$

6

4,5,6,... S*(N-2) (N-2)/N 4.5 stars 5.0 stars 4.5 stars 1.0 stars 3.5 stars 2.0 stars $$$

7

varies varies varies 3.0 stars 4.0 stars 4.5 stars 4.0 stars 4.5 stars 4.0 stars $$$$$

01/10

4,6,8,... S*N/2 50% 4.0 stars 4.5 stars 4.5 stars 3.5 stars 4.5 stars 3.5 stars $$$

03/30

6,8,9,10,... S*N0*(N3-1) (N3-1)/N3 3.5 stars 4.0 stars 4.0 stars 2.0 stars 4.5 stars 3.0 stars $$$$

05/50

6,8,9,10,... S*N0*(N5-1) (N5-1)/N5 3.5 stars 4.0 stars 4.5 stars 3.0 stars 4.0 stars 3.0 stars $$$$

15/51

6,8,10,... S*((N/2)-1) ((N/2)-1)/N 5.0 stars 5.0 stars 4.0 stars 3.0 stars 4.0 stars 3.0 stars $$$$$

Notes on the table:

Strategie di configurazione

Tre fattori guidano l'implementazione delle configurazioni RAID:

La priorità di ognuno di questi fattori determinerà il livello RAID ottimale per un array.

Ottimizzazione della tolleranza agli errori

Livello RAID Descrizione della tolleranza agli errori
0 Non tollerante agli errori. I guasti dell'unità non sono tollerati.
1 Mirroring del disco; copia completa dei dati su un'unità disco rigido secondaria. Un guasto dell'unità può essere tollerato.
5 Parità distribuita e striping; i dati di parità per tutti i dischi sono distribuiti sui dischi rimanenti. Un guasto dell'unità può essere tollerato.
10 Mirroring del disco e striping; viene eseguito lo striping dei dati su tutti gli array di Livello RAID 1. Può essere tollerato un guasto dell'unità per ogni array primario.
50 Parità distribuita e striping; viene eseguito lo striping dei dati su tutti gli array di livello RAID 5. Può essere tollerato un guasto dell'unità per ogni array primario.

Ottimizzazione delle prestazioni

RAID Caratteristiche di prestazione
0 Prestazioni ottimali in lettura e scrittura; senza tolleranza agli errori.
1 Buone prestazioni in lettura e prestazioni inferiori in scrittura.
5 Prestazioni buone o molto buone in lettura, basse prestazioni in scrittura.
10 Prestazioni da buone a eccellenti in lettura, da buone ad alte in scrittura.
50 Prestazioni da buone a eccellenti in lettura, da medie a buone in scrittura.

Ottimizzazione della capacità

Livello RAID Descrizione Unità supportate Capacità
0 stripe 1-30 (numero di dischi) x (capacità minima dei dischi)
1 Mirroring 2 Capacità minima dei dischi.
5 Striping distribuita 3-16 (capacità minima dei dischi) x (numero di dischi - 1)
10 Striping eseguita su livello RAID 1 4-30 (multiplo di 2) (capacità minima dei dischi) x (numero di dischi / 2)
50 Striping eseguita su livello RAID 5 6-30 (Capacità minima dei dischi) x (numero di dischi) - (numero degli array di livello RAID 5)

Livelli RAID

Livello RAID Descrizione Sicurezza Velocità
Scrittura/Lettura
Linear Due o più dischi vengono combinati insieme per formare uno logico. I dati vengono scritti sul primo, una volta pieno si passa al secondo e così via. I dischi o le partizioni, non devono avere le stesse dimensioni. Non è un vero sistema RAID, poichè non implementa nessuna ridondanza e dal punto di vista delle prestazioni non c'è incremento. E' utile per aumentare la capacità di archiviazione. * */*
RAID 0 - Striping E' il livello RAID minimo, ottimo per incrementare le prestazioni. Non è sicuro perchè non viene effettuata nessuna copia dei dati (niente ridondanza) e, quindi, risulta vulnerabile come una normale installazione su singolo disco, benchè più veloce * ***/***
RAID 1 - Mirroring Il primo vero livello RAID. I dati vengono scritti su tutti i dischi/partizioni, mirroring. In caso di guasto basta sostituire l'hard disk non funzionante senza interruzioni del servizio. Le prestazioni in scrittura decadono leggermente, quelle in lettura sono migliori. *** */***
RAID 4 Simile al RAID 0, necessita di almeno 3 dischi. Quello aggiuntivo, chiamato parity disk, contiene dati che servono per ricostruire la partizione non funzionante per problemi hardware (disk failure). Le prestazioni sono influenzate da quelle del disco di parity ** */*
RAID 5 Simile al RAID 4 non utilizza però il disco di parity e i dati per la ricostruzione sono distribuiti equamente sui vari supporti. L'incremento prestazionale è maggiore ** ***/***
RAID 0+1, RAID 10
Mirrored striping
Con almeno 4 dischi possiamo avere un sistema che unisce la velocità del livello 0 con la sicurezza garantita dal livello 1. Indubbiamente è la soluzione migliore. ** ***/***

 


An Introduction to RAID

RAID stands for Redundant Array of Independent Disks. The idea of RAID technology was to use an array of hard disks for either better performance or better security against disk failure. Raid can use 2 or more disks at once to increase data reading and writing speed, It can use 2 or more disks to store the same data so disk failure will not mean that you lose your data, or RAID can be a mixture of both. A RAID Array of disks will appear to an operating system as a single disk as extra storage space is not provided by RAID.

The common RAID functions as mentioned above comes in 3 different levels. These are called RAID 0, RAID 1 and RAID 0+1. This is the terminology you will see when you are buying a motherboard that supports the RAID feature. The real names of these levels are. Stripping, Mirroring and Stripping + Mirroring.

RAID 0 Striping

The Diagram to the left shows the basics of the RAID 0 or striping feature. The idea of RAID 0 is to increase performance. When storing information using the striping feature, the data will be split block by block between the two hard disks. Block one will be send to disk one, block two will be sent to disk two, block three will be sent to disk one and block 4 will be sent to disk 2 and so on. This is much faster than a single disk because when reading the data off the disks the twp of them will be working at the same time to retrieve the same file virtually doubling the speed or retrieval and so virtually halving the time of retrieval. As I mentioned this is a performance setup. Should any one of the disks fail the whole array will become corrupt. Most of the files will be split between disks and so will be rendered useless. If you don't have important data on your computer or you have regular backups of what you do need, then a RAID 0 setup would greatly increase your computers disk performance. To get the best out of this system it is wise to use two disks which are the same make and model. If this is not possible then two of the same size and RPM would be useful but not essential. If two disks of different sizes are used in this system then the logical drive will show as the smallest disk. See drive capacities under RAID at the end of this article.

 

 

 

 

 

RAID 1 Mirroring

RAID 1 or mirroring gives added security for your data at the cost of storage space. As with striping this setup uses two hard disk drives to produce a single logical drive. In this instance however the total storage space is only the size of one of the disks (the smallest one). This is because with RAID 1 any data that is written to or read from the hard disk is done on the second hard disk exactly the same. If you save a file to your machine, it will will saved on both disks at the same time. This will however affect system performance with two disks needing to be written to and with the data being the same, its no better in terms of performance unlike the stripping method. However there is always advantages. Mirroring, having the same data on both disks has obvious plus points when it comes to data integrity and security against disk failure. If either disk one or two should fail the other disk will take over as the solitary disk providing and storing data like it did before the failure. Again see the drive capacity section at the end of this article to learn about data redundancy and why the logical drive sizes are what they are with each of the three RAID setups.

 

 

 

 

 

 

 

 

RAID 0+1 Striping and Mirroring

RAID 0+1 or Striping + Mirroring as you would imagine is a combination of the above two setups. This setup takes the advantages of both the stripping setup and the mirroring setup. You get the increased performance of splitting the data across multiple drives, however each of these striped drives will have a mirror as well for the data backup and security against failure. The obvious drawback here is the cost involved. The minimum amount of hard disk drives used in this configuration is 4. This puts most home users out of the equation as not only do you need to buy 4 hard disks but the PSU has to cope as well.

 

 

 

 

 

 

 

Disk Capacities Using the RAID Function
These RAID functions give you varying capacities for your hard disk. To illustrate this we will take an example of a user using only 80Gb hard disks. We will take each of RAID levels mentioned to see what drive capacities you would get out of them.
RAID 0 Striping
With RAID 0 and using the 2 80Gb hard disks you would get the full 160Gb of storage space. Although the data is split between the 2 hard disks. There is no data redundancy (duplicate data). This allows for the full storage space to be used.
RAID 1 Mirroring
When using two 80Gb hard disks with the RAID 1 function you would only receive 80Gb of storage space. Because you are using the two drives to contain the same data, the logical drive will appear as a single 80Gb drive.
RAID 0+1 Striping
In this example we would need to use 4 80Gb drives. RAID 0+1 is a combination of the two above and so storage works out as a combination of the two as well. The logical drive will appear as a single drive, this drives capacity will be 160Gb. The 2 striped drives will be included in the logical drives space, but as above the mirrored drives will appear invisible to the user.


Livelli RAID

Livello RAID Descrizione Sicurezza Velocità
Scrittura/Lettura
Linear Due o più dischi vengono combinati insieme per formare uno logico. I dati vengono scritti sul primo, una volta pieno si passa al secondo e così via. I dischi o le partizioni, non devono avere le stesse dimensioni. Non è un vero sistema RAID, poichè non implementa nessuna ridondanza e dal punto di vista delle prestazioni non c'è incremento. E' utile per aumentare la capacità di archiviazione. * */*
RAID 0 - Striping E' il livello RAID minimo, ottimo per incrementare le prestazioni. Non è sicuro perchè non viene effettuata nessuna copia dei dati (niente ridondanza) e, quindi, risulta vulnerabile come una normale installazione su singolo disco, benchè più veloce * ***/***
RAID 1 - Mirroring Il primo vero livello RAID. I dati vengono scritti su tutti i dischi/partizioni, mirroring. In caso di guasto basta sostituire l'hard disk non funzionante senza interruzioni del servizio. Le prestazioni in scrittura decadono leggermente, quelle in lettura sono migliori. *** */***
RAID 4 Simile al RAID 0, necessita di almeno 3 dischi. Quello aggiuntivo, chiamato parity disk, contiene dati che servono per ricostruire la partizione non funzionante per problemi hardware (disk failure). Le prestazioni sono influenzate da quelle del disco di parity ** */*
RAID 5 Simile al RAID 4 non utilizza però il disco di parity e i dati per la ricostruzione sono distribuiti equamente sui vari supporti. L'incremento prestazionale è maggiore ** ***/***
RAID 0+1, RAID 10
Mirrored striping
Con almeno 4 dischi possiamo avere un sistema che unisce la velocità del livello 0 con la sicurezza garantita dal livello 1. Indubbiamente è la soluzione migliore. ** ***/***

RAID Types - Classifications

RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. A RAID array is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other.

 
RAID 0 (Striping)
RAID 1 (Mirroring)
RAID 0+1
RAID 2 (ECC)
RAID 3
RAID 4
RAID 5
RAID 6
RAID 7 (Proprietary)
RAID 10
RAID 1E
RAID 50 (same as RAID 05)
RAID 53


The "RAID" acronym first appeared in 1988 in the earliest of the Berkeley Papers written by Patterson, Gibson & Katz of the University of California at Berkeley. The RAID Advisory Board has since substituted "Independent" for "Inexpensive". A series of papers written by the original three authors and others defined and categorized several data protection and mapping models for disk arrays. Some of the models described in these papers, such as mirroring, were known at the time, others were new. The word levels used by the authors to differentiate the models from each other may suggest that a higher numbered RAID model is uniformly superior to a lower numbered one. This is not the case.

RAID 0 (Striping)

RAID-Classes

  RAID 0: Striped Disk Array without Fault Tolerance
RAID Level 0 requires a minimum of 2 drives to implement.

RAID Level 0 is a performance oriented striped data mapping technique. Uniformly sized blocks of storage are assigned in regular sequence to all of an array's disks. RAID Level 0 provides high I/O performance at low inherent cost. (No additional disks are required). The reliability of RAID Level 0, however is less than that of its member disks due to its lack of redundancy. Despite the name, RAID Level 0 is not actually RAID, unless it is combined with other technologies to provide data and functional redundancy, regeneration and rebuilding.

Advantages: RAID 0 implements a striped disk array, the data is broken down into blocks and each block is written to a separate disk drive. I/O performance is greatly improved by spreading the I/O load across many channels and drives. Best performance is achieved when data is striped across multiple controllers with only one drive per controller. No parity calculation overhead is involvedVery simple designEasy to implement.

Disadvantages: Not a "True" RAID because it is NOT fault-tolerant. The failure of just one drive will result in all data in an array being lost. Should never be used in mission critical environments. Recommended Applications� Video Production and Editing � Image Editing � Pre-Press Applications � Any application requiring high bandwidth.

RAID 1 (Mirroring)

RAID-Classes

  RAID 1: Mirroring and Duplexing. For Highest performance, the controller must be able to perform two concurrent separate Reads per mirrored pair or two duplicate Writes per mirrored pair.
RAID Level 1 requires a minimum of 2 drives to implement.

RAID Level 1, also called mirroring, has been used longer than any other form of RAID. It remains popular because of its simplicity and high level of reliability and availability. Mirrored arrays consist of two or more disks. Each disk in a mirrored array holds an identical image of user data. A RAID Level 1 array may use parallel access for high transfer rate when reading. More commonly, RAID Level 1 array members operate independently and improve performance for read-intensive applications, but at relatively high inherent cost. This is a good entry-level redundant system, since only two drives are required.

Advantages: One Write or two Reads possible per mirrored pair. Twice the Read transaction rate of single disks. Same write transaction rate as single disks. 100% redundancy of data means no rebuild is necessary in case of a disk failure, just a copy to the replacement disk. Transfer rate per block is equal to that of a single disk. Under certain circumstances, RAID 1 can sustain multiple simultaneous drive failures. Simplest RAID storage subsystem design.

Disadvantages: Highest disk overhead of all RAID types (100%) - inefficient. Typically the RAID function is done by system software, loading the CPU/Server and possibly degrading throughput at high activity levels. Hardware implementation is strongly recommended. May not support hot swap of failed disk when implemented in "software". Recommended Applications� Accounting � Payroll � Financial � Any application requiring very high availability.

RAID 0+1

RAID-Classes

  RAID 0+1: High Data Transfer Performance
RAID Level 0+1 requires a minimum of 4 drives to implement.

RAID Level 0+1 is a striping and mirroring combination without parity. RAID 0+1 has fast data access (like RAID 0), and single-drive fault tolerance (like RAID 1). RAID 0+1 still requires twice the number of disks (like RAID 1).

Advantages: RAID 0+1 is implemented as a mirrored array whose segments are RAID 0 arrays. RAID 0+1 has the same fault tolerance as RAID level 5. RAID 0+1 has the same overhead for fault-tolerance as mirroring alone. High I/O rates are achieved thanks to multiple stripe segments. Excellent solution for sites that need high performance but are not concerned with achieving maximum reliability.

Disadvantages: RAID 0+1 is NOT to be confused with RAID 10. A single drive failure will cause the whole array to become, in essence, a RAID Level 0 array. Very expensive / High overhead. All drives must move in parallel to proper track lowering sustained performance. Very limited scalability at a very high inherent cost. Recommended Applications� Imaging applications � General fileserver.

RAID 2 (ECC)

RAID-Classes

  RAID 2: Hamming Code ECC Each bit of data word is written to a data disk drive (4 in this example: 0 to 3). Each data word has its Hamming Code ECC word recorded on the ECC disks. On Read, the ECC code verifies correct data or corrects single disk errors.

RAID Level 2 is one of two inherently parallel mapping and protection techniques defined in the Berkeley paper. It has not been widely deployed in industry largely because it requires special disk features. Since disk production volumes determine cost, it is more economical to use standard disks for RAID systems.

Advantages: "On the fly" data error correction. Extremely high data transfer rates possible. The higher the data transfer rate required, the better the ratio of data disks to ECC disks. Relatively simple controller design compared to RAID levels 3,4 & 5.

Disadvantages: Very high ratio of ECC disks to data disks with smaller word sizes - inefficient. Entry level cost very high - requires very high transfer rate requirement to justify. Transaction rate is equal to that of a single disk at best (with spindle synchronization). No commercial implementations exist / not commercially viable.

RAID 3

RAID-Classes

  RAID 3: Parallel transfer with Parity The data block is subdivided ("striped") and written on the data disks. Stripe parity is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 3 requires a minimum of 3 drives to implement.

RAID Level 3 adds redundant information in the form of parity to a parallel access striped array, permitting regeneration and rebuilding in the event of a disk failure. One stripe of parity protects corresponding strip's of data on the remaining disks. RAID Level 3 provides for high transfer rate and high availability, at an inherently lower cost than mirroring. Its transaction performance is poor, however, because all RAID Level 3 array member disks operate in lockstep.

RAID 3 utilizes a striped set of three or more disks with the parity of the strips (or chunks) comprising each stripe written to a disk. Note that parity is not required to be written to the same disk. Furthermore, RAID 3 requires data to be distributed across all disks in the array in bit or byte-sized chunks. Assuming that a RAID 3 array has N drives, this ensures that when data is read, the sum of the data-bandwidth of N - 1 drives is realized. The figure below illustrates an example of a RAID 3 array comprised of three disks. Disks A, B and C comprise the striped set with the strips on disk C dedicated to storing the parity for the strips of the corresponding stripe. For instance, the strip on disk C marked as P(1A,1B) contains the parity for the strips 1A and 1B. Similarly the strip on disk C marked as P(2A,2B) contains the parity for the strips 2A and 2B.

RAID-Classes

Advantages: Very high Read data transfer rate. Very high Write data transfer rate. Disk failure has an insignificant impact on throughput. Low ratio of ECC (Parity) disks to data disks means high efficiency. RAID 3 ensures that if one of the disks in the striped set (other than the parity disk) fails, its contents can be recalculated using the information on the parity disk and the remaining functioning disks. If the parity disk itself fails, then the RAID array is not affected in terms of I/O throughput but it no longer has protection from additional disk failures. Also, a RAID 3 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.

Disadvantages: Transaction rate equal to that of a single disk drive at best (if spindles are synchronized). Read operations can be time-consuming when the array is operating in degraded mode. Due to the restriction of having to write to all disks, the amount of actual disk space consumed is always a multiple of the disks' block size times the number of disks in the array. This can lead to wastage of space. Controller design is fairly complex. Very difficult and resource intensive to do as a "software" RAID. Recommended Applications� Video Production and live streaming � Image Editing � Video Editing � Prepress Applications � Any application requiring high throughput.

RAID 4

RAID-Classes

  RAID 4: Independent Data disks with Shared Parity disk Each entire block is written onto a data disk. Parity for same rank blocks is generated on Writes, recorded on the parity disk and checked on Reads.
RAID Level 4 requires a minimum of 3 drives to implement.

Like RAID Level 3, RAID Level 4 uses parity concentrated on a single disk to protect data. Unlike RAID Level 3, however, a RAID Level 4 array's member disks are independently accessible. Its performance is therefore more suited to transaction I/O than large file transfers. RAID Level 4 is seldom implemented without accompanying technology, such as write-back cache, because the dedicated parity disk represents an inherent write bottleneck.

Advantages: Very high Read data transaction rate. Low ratio of ECC (Parity) disks to data disks means high efficiency. High aggregate Read transfer rate.

Disadvantages: Quite complex controller design. Worst Write transaction rate and Write aggregate transfer rate. Difficult and inefficient data rebuild in the event of disk failure. Block Read transfer rate equal to that of a single disk.

RAID 5

RAID-Classes

  RAID 5: Independent Data disks with Distributed Parity blocks Each entire data block is written on a data disk; parity for blocks in the same rank is generated on Writes, recorded in a distributed location and checked on Reads. The array capacity is N-1.
RAID Level 5 requires a minimum of 3 drives to implement.

By distributing parity across some or all of an array's member disks, RAID Level 5 reduces (but does not eliminate) the write bottleneck inherent in RAID Level 4. As with RAID Level 4, the result is asymmetrical performance, with reads substantially outperforming writes. To reduce or eliminate this intrinsic asymmetry, RAID level 5 is often augmented with techniques such as caching and parallel multiprocessors.

The figure below illustrates an example of a RAID 5 array comprised of three disks - disks A, B and C. For instance, the strip on disk C marked as P(1A,1B) contains the parity for the strips 1A and 1B. Similarly the strip on disk A marked as P(2B,2C) contains the parity for the strips 2B and 2C. RAID 5 ensures that if one of the disks in the striped set fails, its contents can be extracted using the information on the remaining functioning disks. It has a distinct advantage over RAID 4 when writing since (unlike RAID 4 where the parity data is written to a single drive) the parity data is distributed across all drives. Also, a RAID 5 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.

RAID-Classes

Advantages: Highest Read data transaction rate. Medium Write data transaction rate. Low ratio of ECC (Parity) disks to data disks means high efficiency. Good aggregate transfer rate.

Disadvantages: Disk failure has a medium impact on throughput. Most complex controller design. Difficult to rebuild in the event of a disk failure (as compared to RAID level 1). Individual block data transfer rate same as single disk. Recommended Applications� File and Application servers � Database servers � WWW, E-mail, and News servers � Intranet servers � Most versatile RAID level.

RAID 6

RAID-Classes

  RAID 6: Independent Data disks with two Independent Distributed Parity schemes.

Advantages: RAID 6 is essentially an extension of RAID level 5 which allows for additional fault tolerance by using a second independent distributed parity scheme (two-dimensional parity). Data is striped on a block level across a set of drives, just like in RAID 5, and a second set of parity is calculated and written across all the drives. RAID 6 provides for an extremely high data fault tolerance and can sustain multiple simultaneous drive failures. Perfect solution for mission critical applications.

Disadvantages: Very complex controller design. Controller overhead to compute parity addresses is extremely high. Very poor write performance. Requires N+2 drives to implement because of two-dimensional parity scheme.

RAID 7 (Proprietary)

RAID-Classes

  RAID 7: Optimized Asynchrony for High I/O Rates as well as High Data Transfer Rates.

Architectural Features:� All I/O transfers are asynchronous, independently controlled and cached including host interface transfers� All Reads and Write are centrally cached via the high speed x-bus� Dedicated parity drive can be on any channel� Fully implemented process oriented real time operating system resident on embedded array control microprocessor� Embedded real time operating system controlled communications channel� Open system uses standard SCSI drives, standard PC buses, motherboards and memory SIMMs� High speed internal cache data transfer bus (X-bus)� Parity generation integrated into cache� Multiple attached drive devices can be declared hot standbys� Manageability: SNMP agent allows for remote monitoring and management.

Advantages: Overall write performance is 25% to 90% better than single spindle performance and 1.5 to 6 times better than other array levelsHost interfaces are scalable for connectivity or increased host transfer bandwidth. Small reads in multi user environment have very high cache hit rate resulting in near zero access times. Write performance improves with an increase in the number of drives in the array. Access times decrease with each increase in the number of actuators in the array. No extra data transfers required for parity manipulation. RAID 7 is a registered trademark of Storage Computer Corporation.

Disadvantages: One vendor proprietary solution. Extremely high cost per MB. Very short warranty. Not user serviceable. Power supply must be UPS to prevent loss of cache data.

RAID 10

RAID-Classes

RAID 10: Very High Reliability combined with High Performance
RAID Level 10 requires a minimum of 4 drives to implement.

Advantages: RAID 10 is implemented as a striped array whose segments are RAID 1 arrays. RAID 10 has the same fault tolerance as RAID level 1. RAID 10 has the same overhead for fault-tolerance as mirroring alone. High I/O rates are achieved by striping RAID 1 segments. Under certain circumstances, RAID 10 array can sustain multiple simultaneous drive failures. Excellent solution for sites that would have otherwise gone with RAID 1 but need some additional performance boost.

Disadvantages: Very expensive / High overhead. All drives must move in parallel to proper track lowering sustained performance. Very limited scalability at a very high inherent cost. Recommended Applications� Database server requiring high performance and fault tolerance.

RAID 10 arrays are typically used in environments that require uncompromising availability coupled with exceptionally high throughput for the delivery of data located in secondary storage. In recent years a number of mutations of RAID 10 have been developed with similar capabilities. This paper presents one of the popular alternative implementations and discusses the relative advantages and disadvantages of RAID 10 and this alternative.

A RAID 10 array is formed using a two-layer hierarchy of RAID types. At the lowest level of the hierarchy are a set of RAID 1 sub-arrays i.e., mirrored sets. These RAID 1 sub-arrays in turn are then striped to form a RAID 0 array at the upper level of the hierarchy. The collective result is a RAID 10 array. The figure below demonstrates a RAID 10 comprised of two RAID 1 sub-arrays at the lower level of the hierarchy. They are sub-arrays A (comprised of disks A1 and A2) and B (comprised of disks B1 and B2). These two sub-arrays in turn are striped using the strips 1A, 1B, 2A, 2B, 3A, 3B, 4A, 4B to form a RAID 0 at the upper level of the hierarchy. The result is a RAID 10. Figure 1 illustrates a RAID 10 array, with each disk in the array participating in exactly one mirrored set, thereby forcing the number of disks in the array to be even.

RAID-Classes

Let us now look at some of the salient properties of RAID 10. Consider a RAID 10 comprised of d disks and N mirrored sets (i.e., constituent RAID 1 sub-arrays). Since each disk in the array participates in exactly one mirrored set, d = 2N.

(a) RAID 10 arrays do not require any parity calculation at any stage of their construction or operation.

(b) RAID 10 arrays are generally deployed in environments that require a high degree of redundancy. The ability to survive multiple failures is a fundamental property of RAID 10. In fact the maximum number of disk failures a RAID 10 array can withstand is d/2 = N.

What about the number of combinations of failed disks that a RAID 10 array can sustain? The number of ways in which k disks can fail is given by NCk �2k, since there are NCk ways in which to choose k mirror groups from N possible choices, and 2 ways in which to choose a disk within each mirror group. Therefore the total number of combinations of failed disks that a RAID 10 can support is:

NC1 �21 + NC2 �22 + � + NCN �2N
= (2 + 1)N - 1
= 3N - 1

Thus, for a 4 drive RAID 10 containing 2 mirrored sets, the number of combinations in which disks can fail without the array being rendered inoperable is 32 - 1 = 8. In fact, these combinations may be enumerated as follows, with each possible set of failed disks listed within braces. They are: {A1}, {A2}, {B1}, {B2}, {A1, B1}, {A2, B2}, {A1, B2}, and {A2, B1}.

(c) RAID 10 ensures that if a disk in any constituent mirrored set fails, its contents can be extracted from the functioning disk in its mirrored set. Thus, when a RAID 10 array has suffered the maximum number of disk failures it is capable of withstanding, its throughput rate is no worse than that of a RAID 0 with N disks. In fact, any combination of N contiguous independent strips can be read concurrently. The term "independent strip" is used to denote a strip in a collection of strips that is not a mirror of any other strip within that collection.

(d) A RAID 10 array that is in a nominal state can improve the throughput of read operations by allowing concurrent reads to be performed on multiple disks in the array. For example, if the strips 1A, 1B, 2A, 2B are to be read from the array given in figure 1, it is clear that all four strips can be read concurrently from the disks A1, B1, A2 and B2 respectively.

RAID 1E

RAID-Classes

  RAID 1E: While RAID 10 has been traditionally implemented using an even number of disks, some hybrids can use an odd number of disks as well. Figure 2 illustrates an example of a hybrid RAID 10 array comprised of five disks; A, B, C, D and E. In this configuration, each strip is mirrored on an adjacent disk with wrap-around. In fact this scheme - or a slightly modified version of it - is often referred to as RAID 1E and was originally proposed by IBM. Let us now investigate the properties of this scheme.

When the number of disks comprising a RAID 1E is even, the striping pattern is identical to that of a traditional RAID 10, with each disk being mirrored by exactly one other unique disk. Therefore, all the characteristics for a traditional RAID 10 apply to a RAID 1E when the latter has an even number of disks. However, RAID 1E has some interesting properties when the number of disks is odd.

(a) Just as in the case of traditional RAID 10, RAID 1E does not require any parity calculation either. So in this category, RAID 10 and RAID 1E are equivalent.

(b) The maximum number of disk failures a RAID 1E array using d disks can withstand is d/2 . When d is odd, this yields a value that is the equal to that of a traditional RAID 10 while utilizing one additional disk. What about the number of combinations of disk failures that RAID 1E can support? It turns out that RAID 1E is very peculiar in this characteristic when d is odd. Assume for the sake of notational convenience that d/2 = p. Then the number of ways in which k disks can fail is d�P-1Ck-1, since there are d ways to choose the first disk and P-1Ck-1 ways to choose the remaining k-1 disks from p-1 possible choices. Therefore, the total number of combinations of failed disks that this scheme can support is:

d�p-1C0 + d�p-1C1 + ... + d�p-1Cp-1
= d � (p-1C0 + p-1C1 + � + p-1Cp-1)
= d � 2p-1

Thus, for a 5 drive RAID 1E, the total number of combinations in which disks can fail without the array being rendered inoperable is 5�22-1 = 10. However, this result also indicates that as the value of d increases, the ratio of the number of combinations of disk failures supported by RAID 1E using d disks decreases with respect to conventional RAID 10 using d-1 disks. In fact, for d > 9, RAID 1E yields a lesser number of combinations! For instance, while a conventional RAID 10 using 10 disks can support 35 - 1 = 242 combinations of disk failures, RAID 1E using 11 disks can support only 11�25-1 = 176 combinations. Clearly, RAID 10 is a superior choice when tolerance to a larger number of combinations of disk failures is considered important. An even more significant implication of this result is the following. Since a RAID 1E with an even number of disks is identical to a traditional RAID 10, A RAID 1E with 10 disks can support more combinations of failures than a RAID 1E with 11 disks. In general, a RAID 1E with 2N disks can support more combinations of failures that a RAID 1E with 2N + 1 disks, when N 5. In other words, it is always preferable to utilize an even number of disks for your RAID 1E than an odd number if you desire a higher tolerance to disk failures. In other words, it is always preferable to use a traditional RAID 10!

(c) When a RAID 1E array suffers the maximum number of disk failures it is capable of withstanding, i.e., d/2 , the number of contiguous independent strips that can be accessed concurrently can be less than d/2 . For example, consider the RAID 1E array displayed in figure 2. Assume that disks A and C have failed. In this scenario, it is clear that the contiguous strips 4, 5 and 6 cannot be read concurrently although three disks remain operational. Thus the throughput of a RAID 1E with d disks - where d is odd - may be no higher under specific access patterns than that of a RAID 10 with d-1 disks when both arrays experience the maximum number of sustainable disk failures.

(d) Just as in the case of a traditional RAID 10 implementation, RAID 1E in a nominal state can improve the throughput of read operations by allowing concurrent reads to be performed on multiple disks in the array. The fact that there are more disks than there are mirror sets should intuitively suggest as much.

Conclusion: RAID 1E offers a little more flexibility in choosing the number of disks that can be used to constitute an array. The number can be even or odd. However, RAID 10 is far more robust in terms of the number of combinations of disk failures it can sustain even when using lesser number of disks. Furthermore, a RAID 10 guarantees a throughput rate that is always equal to that which is obtainable from the concurrent use of all its functioning disks. In contrast, specific access patterns may not lend themselves to the concurrent use of all functioning disks under RAID 1E. Therefore, if extremely high availability and throughput are of paramount importance to your applications, RAID 10 should be the configuration of choice.

RAID 50 (same as RAID 05)

RAID-Classes

  RAID 50 array is formed using a two-layer hierarchy of RAID types. At the lowest level of the hierarchy is a set of RAID 5 arrays. These RAID 5 arrays in turn are then striped to form a RAID 0 array at the upper level of the hierarchy. The collective result is a RAID 50 array. The figure below demonstrates a RAID 50 comprised of two RAID 5 arrays at the lower level of the hierarchy � arrays X and Y. These two arrays in turn are striped using 4 stripes (comprised of the strips 1X, 1Y, 2X, 2Y, etc.) to form a RAID 0 at the upper level of the hierarchy. The result is a RAID 50.

Advantage: RAID 50 ensures that if one of the disks in any parity group fails, its contents can be extracted using the information on the remaining functioning disks in its parity group. Thus it offers better data redundancy than the simple RAID types, i.e., RAID 1, 3, and 5. Also, a RAID 50 array can improve the throughput of read operations by allowing reads to be performed concurrently on multiple disks in the set.

RAID 53

RAID-Classes

  RAID 53: High I/O Rates and Data Transfer Performance
RAID Level 53 requires a minimum of 5 drives to implement.

Advantages: RAID 53 should really be called "RAID 03" because it is implemented as a striped (RAID level 0) array whose segments are RAID 3 arrays. RAID 53 has the same fault tolerance as RAID 3 as well as the same fault tolerance overhead. High data transfer rates are achieved thanks to its RAID 3 array segments. High I/O rates for small requests are achieved thanks to its RAID 0 striping. Maybe a good solution for sites who would have otherwise gone with RAID 3 but need some additional performance boost.

Disadvantages: Very expensive to implement. All disk spindles must be synchronized, which limits the choice of drives. Byte striping results in poor utilization of formatted capacity.

I vantaggi della tecnologia RAID


Setting up a RAID Array in XP/2000

Click here to download a printable version of RAID set up

Here is a full description of how to install RAID. (You Should Cut and Paste this to word and print it)
When the drives arrive and you have installed them to their SATA connections:
1) Get your Motherboard Manual
2) Start PC and enter BIOS
3) Look in manual and find where you enable RAID on the motherboard
4) Enable it
5) Exit and save to CMOS
6) After BIOS Post, the RAID BIOS will show. Press the listed keys to enter the RAID Configuration BIOS (your motherboard will show you what keys to press)
7) Listed in the RAID BIOS will be several options:
-Automatically configure a RAID setup (some RAID controllers only)
-Manually set up RAID Array
-Delete RAID Array
-Exit
8) Select manually set up RAID Array
9) Make these choices:
-Select RAID 0
-64K Stripe Size (64k is the best performing size for all applications, especially video files and gaming)
-Select the drives
-Select the Array to be a Boot Device (only available on some RAID Controllers)
-Save
-Place Windows CD in CD Drive
-Exit RAID Set up utility
10) After boot, windows will begin to load, at the first blue screen; it will ask you to press F6 to load RAID drivers. Press F6
11) Once windows finishes placing info to memory it will show the screen to load the drivers for RAID. It will ask you to press "S" to load new storage device, press "s"
12) A new screen will ask you to place floppy in and press enter, do this
13) It will then show the drivers, press enter to select
14) The previous driver entry screen will appear showing your selected RAID drivers; press enter to allow windows to finish loading its file to memory
15) Continue entering a clean install of windows and select your partition size (Best is 32GB for each partition)
16) Format partition with FAT32 and load windows to c: (once drive is formatted and windows loads files and wants to reboot, remove floppy)
17) Allow windows to boot and begin proper install
18) Once it boots into windows desktop you can begin regular installation of your programs, no other extra steps are needed.

The Top Five RAID Tips from Promise
Advice on configuration from the RAID experts.
Promise pioneered the use of low-cost ATA drives in RAID arrays, enabling high-end performance at one-half to one-third the cost of SCSI drives. If you’re already using ATA RAID, then you know it’s a cost-effective solution for kicking up storage system performance. And, if you’re into maximum performance, you’re probably using RAID 0. Storage system performance is directly related to the hardware you use, but there are still some good rules that apply to all hard drives. Here are some tips from Promise ( www.promise.com ) that should ensure you’re getting the most out of your RAID 0 storage.
(1) When using P-ATA drives, configure the drives as Master drives, with one drive per channel. Using Slave drives will increase overhead, reducing performance.
(2) Use the RAID 0 array as a data drive and not as the boot drive. When the operating system and the page file reside on the boot drive, it creates overhead that can diminish the performance of applications like Adobe’s Photoshop and Premiere.
(3) Using the page file on RAID 0: If you are using multiple RAID 0 arrays—either four drives in Master/Slave (ATA) or four drives as Master on a SATA RAID controller, we recommend the following:
> Boot to onboard IDE
> Use array 1 for page file (experienced users only)
> Use array 2 for the data drive, applications, games, and more
(4) When using Windows XP, turn off the System Restore on the RAID 0 array. System Restore monitors the disk (array), reducing performance.
(5) Use single partitions. Using multiple partitions reduces application performance because you are running I/O to more than one partition on the same disk (array). If you use multiple partitions, do not move the page file to any of the “partitions” on the array where your applications or games are running—this will prevent you from getting optimal performance.
In conclusion, we have found the speed of the RAID 0 to be a welcome addition to our systems and we look forward to the addition of SATA Generation 2 (300Mbps) in the coming year.
Setting up a Promise RAID Card:  1) Best Block (stripe) size for Promise controllers: 64K or 32K
2) After install of OS, go to www.promise.com and download the P.A.M. (promise array manager) and install it but with one change, during install, select custom install not complete, then two pages later you can uncheck the LAN monitoring part, then complete the install
3) Open P.A.M. and navigate to your array as shown Here and at this page, change the cached setting from write through to write back. Press Submit and exit program and reboot.
4) Turn off System restore and defrag all partitions twice, now test your array.
The reason you need to do this is, Promise by default has caching enabled, this will slow down your array, they do this because it is the most stable. Intel however, has caching disabled, this is why some find the Intel faster.


RAID Information - Linux RAID-5 Algorithms
 

This page provides information on the layout of the segments in the various RAID-5 structures in Linux.

Linux RAID-5 Algorithms

Software RAID-5 under the LINUX operating system can use one of four algorithms for the placement of segments among the disks in the array. These will be individually shown below:

Left Asynchronous

In this layout, the segments are numbered sequentially, starting with the first non-parity drive in the stripe. The parity drive starts at the left-most drive, and moves right one drive per stripe. This is the 'standard' RAID-5 layout. It is not the default for Linux.

Left Synchronous

In this layout, the segments are numbered sequentially, starting with the first drive in the stripe after the parity. The segments wrap. The parity drive starts at the left-most drive, and moves right one drive per stripe. This is the default RAID-5 segment layout under Linux.

For large reads, this segment layout is the fastest. This is because each consecutive group of segments that is no larger than the total number of disks in the array, will use all the disks in the array.

Right Asynchronous

In this layout, the segments are numbered sequentially, starting with the first non-parity drive in the stripe. The parity drive starts at the right-most drive, and moves left one drive per stripe.

Right Synchronous

In this layout, the segments are numbered sequentially, starting with the first drive in the stripe after the parity. The segments wrap. The parity drive starts at the right-most drive, and moves left one drive per stripe.


Raid 5
Si può iniziare con 3 dischi. E' importante che che le capacità siano simili, perchè, di fatto, si sfrutterà la capacità del disco più piccolo moltiplicata per il numero di drive. (RAID 5 con Windows XP, bisogna scaricare i programmi SoftRAID Patch, Windows file protection Switcher). La modalità RAID 5 è la migliore se si dispone di un minimo di 3 dischi: Se uno dei dischi si guasta, infatti, i dati presenti su di esso possono essere recuperati in base alle informazioni di parità degli altri drive. inotlre, grazie al processo di memorizzazione in striping, la soluzione Raid 5 assicura una velocità molto elevata che, a seconda del numero di dischi, permette da un punto di vista teorico tempi di lettura da tre a quattro volte inferiori rispetto ad un singolo disco fisso. Il normale bus Parallel Ata (EIDE), con drive più recenti, viene abbastanza caricato già da uno stripe set costituito da due dischi, per cui sarebbe meglio adottare, se possibile una configurazione Serial Ata. Per approfittare dei vantaggi di Raid 5 in Windows XP non occorre acquistare un controller: si può infatti procedere esclusivamente via software.
Sostituzione di tre file di windows
Per prima cosa bisogna copiare sul desktop i file Raidpatch.exe (softRAID Patch) e WFPS10.exe (Windows File Protection Switcher). Avviare innanzitutto WPFS10.exe, che rimuove la protezione contro la copia dei file di sistema di Windows. Rispondere in modo affermativo a tutte le domande che verranno poste. A questo punto, avviare il file RaidPatch.exe. Nel menù che compare verrà richiesto di sostituire tre file di sistema di windows con le copie fornite dal programma softraid: bisogna indicare esplicitamente le cartelle di destinazione. I file DMCONFIG.DLL e DADMIN.EXE vanno posizionati nella cartella System32, mentre il file DMBOOT.EXE va inserito nella sottocartella drivers. Dopo la sostituzione dei file riavviare Windows XP.
Creazione dei supporti dinamici.
Per attivare la modalità Raid 5 per l'array dei drive è necessario convertire il disco fisso di sistema (di solito C) in un cosiddetto supporto dati dinamico. Quest'operazione è consentita dagli strumenti di amministrazione di Windows XP (Start/Pannello di controllo/Strumenti di amministrazione/Gestione computer/Gestione Disco). Fare clic destro su Disco 0 e selezionare converti in disco dinamico. Dopo la conversione si apre una finestra di selezione in cui si può attivare, in base al disco appena convertito, il nuovo array RAID 5. Nel menù successivo spostare i tre dischi Raid 5 dalla finestra Available nella finestra Selected Utilizzando il pulsante Add, poi fare clic su Avanti. Infine si deve assegnare una nuova lettera di drive all'array Raid 5. Ovviamente, non saranno disponibili le lettere precedenti alla conversione (per esempio C, D, E se si usavano tre dischi). Un riavvio conclude l'installazione del sistema RAID.
Ripristino di un disco danneggiato.
Se durante il funzionamento si dovesse verificare l'avaria di un disco, grazie al Raid 5 si potranno recuperare i dati senza perdite. Dopo la sostituzione del disco guasto con uno nuovo e il riavvio di Windows, tornare a gestione disco (punto precedente) e fare clic sul nuovo drive appena installato; nel menù che segue scegliere Riparazione volume. Al termine dell'operazione l'array Raid 5 funzionerà come prima.


RAID Terminology Explained

Hard disks are mechanical devices with moving parts, and as such will break down eventually, compromising any data stored on them that is not backed up. One technology that was developed to deal with this pair of issues is RAID (Redundant Array of Inexpensive Disks).

The idea is to use multiple hard disks in the same system to provide both increased performance (by dividing up data so multiple disks can process different parts of it at the same time) and increased reliability by writing the same information to multiple disks at once.

This technology filtered down to the enthusiast level a while ago, and has become a common feature on many motherboards, as well as an integral part of newer operating systems such as Windows 2000 and XP professional.

In this guide, we will explore how the different implementations of RAID technology function, and how you can make your own RAID setup using a hardware RAID controller, or the software RAID function built into Windows XP Professional.

What is RAID?

RAID, or Redundant Array of Inexpensive Disks, is a technology that uses multiple hard drives to increase the speed of data transfer to and from hard disk storage, and also to provide instant data backup and fault tolerance for any information you might store on a hard drive.

The hard drives are joined in an array (a single logical drive, as far as the operating system is concerned) and all disks share the data written to them in some form. There are several different implementations, or 'levels' of RAID, ranging from RAID 0 to RAID 53.

The common factor that all RAID levels share is the use of a hardware or software RAID controller that intercepts data intended for storage on the logical hard drive. "Logical" being the hard drive space that the operating system sees as a drive letter, C:\ for example.

This data is then either duplicated by the controller for storage on multiple drives in the array at once ('mirroring'), or broken down into smaller chunks which are then divided between the available drives in the RAID array ('striping'). The terminology that is going to be important to understand from here on in is:

RAID array: A group of hard drives linked together as a single logical drive. Must be connected to one or more hardware RAID controllers, or be attached normally to a computer using a RAID capable operating system, such as Windows XP Professional.

Striping: A procedure in which data sent to a RAID array is broken down and portions of it written to each drive in the array. This can dramatically speed up hard drive access when the data is read back, since each drive can transfer part of the data simultaneously.

Mirroring: A procedure in which data sent to a RAID array is duplicated and written onto two or more drives at once.

By breaking down the data and sharing it amongst two or more drives, higher performance can be achieved, especially when reading data back, as each drive can transfer its portion of the required data simultaneously. Of course, striping data on two or more drives actually reduces reliability, since if a single drive in the array fails, all data is lost as each physical hard disk only contains a fragment of the data which is useless without the rest. To combat this problem, a third RAID technology is used called Parity.

Hardware or software RAID?

What is better, hardware or software RAID? Good question.

It really depends on your means and expectations. Software RAID setups through an operating system are inherently lower in performance than hardware RAID controllers, due to the lack of dedicated hardware. They also are, in the case of Windows XP Pro at least, much easier to set up and much more flexible in terms of disk use than a hardware based system.

A second factor to consider is whether you want your operating system disk to be part of the RAID array you create? A major limitation of the WinXP RAID implementation is that the operating system must be installed before a RAID array can be created. This means that if you would like to stripe your operating system disk for increased loading speed, you are out of luck unless you go with a hardware RAID controller.

So to cut it short, if you want the maximum benefit out of creating a striped drive, or need to create a RAID 1 mirror for backups, invest in a motherboard with an on board RAID controller or a PCI add-on controller card. If you want to experiment with striped drives for speed, go with the software solution provided by Windows 2000 or XP as it is easier and cheaper.

How to set up Software RAID in windows XP Professional

Like most other hard drive and storage options, RAID is managed through Windows XP's disk management window, found by right clicking on 'my computer,' then selecting 'manage' followed by 'disk management.' Windows XP Professional is only capable of creating RAID 0 striped arrays, while the various Windows Server operating systems can also create software RAID 1 mirror arrays.

Creating a striped RAID array in XP:

For the purpose of this section of the guide we installed two blank 17GB hard drives on a test system. To create a striped array you must first have at least two drives with a portion of 'unpartitioned space' free. The largest stripe you can create will be twice the size of the smallest unused space on either of the disks. If you have two disks, one with 4GB of unpartitioned space and one with 3GB, the largest striped array you could create would be 6GB, as the area of space used by the stripe on each disk must be the same.

The first step is to convert both disks from basic to dynamic disks within Windows. A dynamic disk is a disk that contains an additional database of other dynamic disks on the system. Dynamic disks can only be read by Windows 2000, XP Professional and the various Windows Server operating systems, and are required to create software RAID arrays within Windows.

For more detail on this subject, see PCstats' Guide to the little known features of Windows XP.

To convert the disks from basic to dynamic, right click the grey box on the left that contains the disk names (disk 1, disk 2, etc.) and select 'convert to dynamic disk…'

From the next Window you can check both blank drives and click 'ok' to convert them.

Once both disks are listed as dynamic, right click the 'unpartitioned space' of either drive and select 'new volume.' On the next page we'll set these drives to be striped, and configure the software RAID options.

Setting up a hardware RAID array

In the 'select volume type' Window, select 'striped.'

Add all disks you wish to use, then decide on the amount of space on both disks you wish to use for the striped volume you are about to create. If you wish, you use only part of each disk for the stripe, leaving the rest free for other uses.

Choose a drive letter or folder to use, and the method of formatting, and you are done. The striped array will format and be ready for use.

Configuring Promise RAID

Note that for the purposes of hardware RAID 0 (striping) it is strongly recommended that you use two disks of the exact same model. For mirror (RAID 1) setups, this is not so essential, but the two drives should be of the same capacity.

Attach the drives to the RAID controller, one drive per channel, set as master for the best performance, and boot the computer. Note that while you can attach both drives to a single IDE port on your RAID controller, you will tend to get better performance with a pair of drives if you plug one into each port during startup, the RAID controller drive detection and setup screen will appear.

Press <CTRL-H> or other key combination as instructed to enter RAID setup.

For Promise RAID controllers

From the main menu, press '1' to enter Auto Setup. From here, you can choose either a RAID 0 or 1 configuration, referred to in this case as either 'performance' or 'security.' Note the separate drive configurations in the screen shots.

Choose and accept the desired RAID type. If you select a stripe (RAID 0) array, no further configuration is necessary. Accept the change and reboot.

If you elect to setup a RAID 1 (mirror) configuration, you must then choose whether you wish to simply create a mirror array (if you have two blank disks and want them to be exact copies when adding data in the future), or create the array and then copy the contents of one disk to the other (if you have a data drive and you wish to create a mirror copy of it for redundancy).

If you elect to mirror and copy data, you will be asked to choose a source drive for the data.

BE CAREFUL. Choosing the wrong drive here can be disastrous, so ensure that you know which drive is which. Paying attention to which port you plugged each drive into should help here, as they will be labeled on the motherboard or card. Once you have created the array, reboot.

Configuring Highpoint RAID controllers

From the main menu, choose 'create RAID'. Press ENTER on the first menu item, 'array mode' and choose either RAID 0 or RAID 1.

The second item, 'select disk drives' lets you specify which drives are included in the array, and if you are using a mirror, lets you choose which drive will be the master in the array. Press ENTER to begin, then press ENTER again to select each drive.

If you are creating a mirror, the first drive you choose will be the master drive and the second will be the mirrored drive. Once you have chosen both, you will be asked whether you wish to duplicate the master drive to the mirror drive now, or simply create the mirror without copying data. Assuming you are using two blank drives, choose the latter option.

For the third item 'block size' accept the default value.

Press ENTER on the fourth item 'start creation process' to create your array. Once you are back to the main menu, press ESC to exit. Your system will reboot.

Initializing and installing (both controllers)

Once Windows loads back up, go to the disk management window. You should be prompted to initialize a new disk. You must do this before Windows XP can access your RAID array. Once you have initialized it, you can right click on the new disk in the disk management lower pane to create a new volume on it and format it in the normal way. You can now use your new RAID volume just like any other drive on your system. As far as windows is concerned, the two disks in your array are one.

Using a hardware RAID array as your system drive

Unlike software RAID arrays, it is actually possible to install Windows or other operating systems onto a hardware array. In the case of Windows, this requires that you have the necessary drivers for your RAID controller on a floppy disk. All hardware controllers should come with this disk; it's the only time you will ever see a driver on a floppy disk these days!

Note that you must have already set up your RAID array through the controller before you attempt to install Windows. Right at the start of the automatic install process for Windows 2000 or XP, as soon as the blue screen appears, you will see a prompt at the bottom of the screen asking you to "Press F6 if you need to install a third party SCSI or RAID driver..."Press F6. Nothing will visibly happen, but after the installation files are copied from the CD, you will see an extra screen for the loading of storage device drivers.

Press 'S' to 'specify an additional device.' You will be prompted to "Please insert the disk labeled Manufacturer-supplied hardware support disk into Drive A:"

Do so and hit enter. After reading the disk, the correct driver for your controller should be shown on screen. Select it and press enter, then enter again to confirm the choice. Windows will then continue to install as normal. And I bet you thought it would be difficult didn't you? Next up, tests to show you just what RAID can do in the performance corner.

How to set up hardware RAID:

For this section, we used a Highpoint HPT 372 ATA/133 RAID controller built into an Epox EP-8K5A2+ motherboard. The drives we used to test our RAID configuration were a pair of Seagate Barracuda ATA 5 7200RPM 120GB hard disks. We also set up a second hardware RAID configuration on a Promise 20276 ATA/133 RAID controller built into an MSI KT3 Ultra2 motherboard, attached to the same pair of 17GB drives used in the software RAID setup above. These two controllers are typical of hardware RAID solutions found on modern motherboards and add-in PCI cards.

We wanted to include instructions for both Highpoint and Promise controllers, as these two companies dominate the home desktop and enthusiast market for RAID controllers. Most RAID setup functions are standard, so if you do not have the same exact controller, these instructions should still translate well.

The following instructions assume two identical blank hard disks. It also assumes that you have correctly installed the Windows drivers for your RAID controller. We used the most recent BIOS versions for both controllers, and we recommend that you obtain these from the manufacturer's website if you have not done so already

Parity and Common types of RAID

In the majority of RAID implementations, a whole drive, or an area of one or more of the drives in the array is dedicated to storing parity information. Essentially, each time a bit of information (a digital 1 or 0) is written to every drive in a striped RAID array, an additional parity bit is generated and stored. The value of this bit is based on whether the total of the bits written to the striped drives is odd or even.

For example, take a three disk RAID array, in which two drives are striped together to hold data, and the third drive is dedicated to storing the parity information. Each time a bit of data is written to each of the data drives, an additional parity bit is written to the parity drive. For argument's sake, let's say that if the value of the two data bits is even (0 and 0 or 1 and 1) then the value of the parity bit would be 0, and if the value of the two bits is odd (0/1, 1/0) then a 1 would be written.

In this way, if one of the data drives fails, a new drive can be added and by comparing the information present on the surviving data drive with the corresponding parity information from the parity drive, the missing information can be written onto the replacement drive a bit at a time.

If any given bit from the parity drive has a value of 1, then we can see (from the values we laid out above) that the total value of the corresponding data bits must be odd. So by looking at the bit value from the surviving drive, we can determine if the value that needs to be written to the replacement drive should be a '1' or a '0.'

RAID technology began as a method to provide additional data security to business servers, and many of the RAID levels are still almost exclusively used in the business domain, due to the cost of the required hardware. Since the lower levels of RAID are easily implemented on modern computers and need only a pair of drives and a RAID-capable drive controller (hardware) or operating system (software), RAID 0 and RAID 1 implementations have become common in the high end desktop/PC enthusiast market.

RAID 0 is used to gain additional performance from conventional drives by pairing them up, while RAID 1 provides a very simple and effective form of backup by duplicating or 'mirroring' all data on a second drive.

Some common Types of RAID

Most Hardware RAID controllers intended for the enthusiast or small business markets support only three levels of RAID; RAID 0, 1 and 0+1. These are the only levels of RAID that do not require the use of parity, as this feature adds greatly to the complexity and expense of the controller.

RAID 0 uses multiple hard drives to stripe data over one large logical drive. While there are physically two drives, the computer logically sees just one. The RAID 0 configuration is typically used when there are data-intensive applications because it offers the fastest data access, though no redundancy.

Generally speaking, software RAID will not support parity, limiting it to the above three levels of RAID. This is the case with Windows XP Pro.

Raid 0: Striped array without fault tolerance

RAID 0 is the most common 'enthusiast' implementation, and the main reason why hardware RAID controllers have found their way onto desktop motherboards from the back corridors of server rooms and IT departments. The attraction is that RAID 0 can essentially combine two hard drives into one using striping, and greatly increase the speed that the drives transfer data.

RAID 0 requires a minimum of two physical drives, but has the advantage of not requiring a parity drive or using space for parity on any of the disks in the array, allowing their full capacity to be used for data.

Of course, this has one obvious disadvantage. There is no fault tolerance. Period.

In fact, technically the reliability of the logical drive created by this form of RAID array is halved for each physical drive present, since if any drive fails, all the data is lost...

 

This is not as big a deal as it may sound, since modern drives generally last at least 2-5 years of constant use, and the performance gains make RAID 0 an excellent choice as the operating system and software drive for your computer. Crucial data is best backed up elsewhere however, like on a RAID 1 configuration for instance.

RAID 1 and RAID 0+1 Explained

RAID 1: Mirrored Disk Array A mirrored disk array is composed of a set of two physical hard drives, each of which contains a full copy of all data sent to the logical drive that represents the array. This has a couple of advantages; first of all, any data stored on a RAID 1 array is completely and automatically backed up, and in the event of the failure of one drive, the other can be substituted without a hitch. Secondly, data can be read from both drives simultaneously, increasing the speed of data retrieval.

Fault tolerance is the cornerstone of RAID 1. In this configuration, two identical physical drives are used, with one drive mirroring the information on the other. A RAID 1 configuration is ideal for data redundancy, though storage is more costly as only 1/2 the total drive space of both hard drives is available.

Data writes take just as long as usual however. In the event one of the drives in the array fails, a new drive can be added, the array rebuilt, and the RAID controller will duplicate the information onto the new blank drive.

The disadvantage of RAID 1 is that unlike striping, a mirrored array can use only half of its total free space for storage, since one disk is an exact duplicate of the other.

RAID 0+1 Striped array with mirroring

This RAID level combines the best features of RAID 0 and 1. It requires a minimum of four physical drives to implement, so it is not cheap. Essentially, two pairs of striped drives are mirrored together to provide fault tolerance. The mirroring provides the fault tolerance, though if any drive is lost, it must be immediately replaced and the array rebuilt, since it cannot handle the loss of more than one drive.

RAID 0+1 does retain the inherent disadvantage of mirroring, however; effective disk space is halved since two of the four drives are exact duplicates of the other pair. Many other implementations of RAID exist, nearly all sharing one common factor: The expense and complexity of the hardware controllers required to implement them.

Intended for business use, these levels of RAID use the parity system as explained above to provide varying levels of fault tolerance. RAID solutions at this level generally come as an add-in controller card or a dedicated storage rack and are intended to work hand-in-hand with hot-swappable hard drive mountings. With this setup, any failed drives can be swapped out for new ones on the fly, and the missing data quickly restored by using the parity data.

Many setups will perform this operation automatically while still maintaining close to normal operation


FAQ sul RAID

Quali livelli RAID supporta Windows NT Server e quali sono le differenze?

La sigla RAID in passato stava per "Redundant Array of Inexpensive Disks", ma con l'arrivo di Windows 2000 RAID ora sta per "Redundant Array of Independent Disks".

Windows NT Server supporta il RAID1 (mirror del disco) e RAID5 (stripe set con parità). Inoltre supporta il Raid 0 (Stripe Set senza parità) ma non è una tecnologia fault-tolerance.

RAID 0 – Stripe Set senza parità

La caratteristica di questa tecnologia è quella di dividere i dati in blocchi da 64k e di redistribuirli uniformemente su tutti i dischi nell'array. Non è considerato un metodo fault tolerant.

RAID 1 – Disk Mirroring e Disk Duplexing

La tecnologia del mirroring permette di duplicare una partizione in un altro disco fisico compresa la partizione di boot.

RAID 5 – Striping con parità

Suddivide i dati e le informazioni di parità in modo uniforme su tutti i dischi. Migliori prestazioni in lettura rispetto alla tecnologia del mirroring.

Windows NT Workstation supporta le tecnologie RAID?

Windows NT Workstation non supporta le tecnologie RAID fault-tolerant (Raid1 o Raid5) ma supporta il Raid 0 (strip set senza parità). Pur avendo il nome di Raid 0 lo stripe set è considerata una tecnologia di ottimizzazione del disco e non fornisce capacità di fault-tolerance.

Come si crea un Mirror Set (Raid1) in ambiente Windows NT/2000?

Windows NT

Per creare un mirror set con lo strumento Disk Administrator, è necessaria una partizione primaria ed uno spazio su disco ancora da partizionare.

1. Dal menu start, selezionare programs, Administrative Tools e Disk Administrator.
2. Selezionare l'esistente partizione primaria tenendo premuto il tasto Ctrl.
3. Selezionare lo spazio non partizionato.
4. Dal menu Fault Tolerance, seleziona Establish Mirror.
5. Dal menu Partition, seleziona Commit changes now per avviare la creazione del mirror.

Windows 2000

Microsoft Windows 2000 introduce dischi base e dischi dinamici. Per utilizzare le tecnologie Raid in Windows 2000 è necessario convertire i dischi base in dinamici.

Per creare il RAID 1 in Windows 2000:

1.Tasto destro su Risorse del computer, seleziona Gestisci.
2.Viene visualizzata la console MMC completa degli Snap-in predefiniti.
3.Espandi il ramo Archiviazione e seleziona Gestione Disco.
4.Seleziona il disco che includerà il mirror, clic su Aggiungi Mirror.
5.Effettuando il mirroring della partizione di boot viene visualizzato un messaggio che informa la creazione del file boot.ini

Come creo uno Stripe Set con parità (Raid 5) in Windows NT Server?

Windows NT

1. Dal menu start, selezionare programs, Administrative Tools e Disk Administrator.
2. Seleziona almeno 3 aree di spazio libero su 3 differenti hard disk.
3. Dal menu Fault Tolerance, seleziona Create Stripe Set with Parity.
4. Inserisci la dimensione dello spazio su disco per creare lo stripe set.

Nota: uno stripe set con parità utilizza lo spazio minimo comune di ogni disco. Esempio: 3 hard disk con 200Mb, 100Mb e 60Mb di spazio libero. Lo spazio minimo è di 60MB per ogni disco con un massimo di 180Mb di partizione totale).

Windows 2000

Per creare il RAID 5 in Windows 2000:

1. Tasto destro su Risorse del computer, seleziona Gestisci.
2. Viene visualizzata la console MMC completa degli Snap-in predefiniti.
3. Espandi il ramo Archiviazione e seleziona Gestione Disco.
4. Seleziona lo spazio su disco non ancora allocato, pulsante destro,seleziona Crea Volume.
5. Viene aperta il wizard Crea Volume. Seleziona Volume Raid 5.
6. Nel pannello di sinistra seleziona i dischi che vuoi utilizzare per creare il Raid 5 e clic su aggiungi. (devi selezionare almeno 3 dischi)
7. Seleziona la dimensione di ogni disco.
8. Seleziona un lettera del driver da utilizzare.
9. Seleziona il file system e l'etichetta. E' possibile abilitare la compressione di file o cartelle.
10. Il programma visualizzerà il resoconto delle operazioni. Clic su termina.


BACKUP:
Esitono vari metodi per preservare i dati. I due metodi maggiormente usati sono il type backup e il fault tolerant disk.
Il processo di type backup è la duplicazione di tutti i dati su un supporto a nastro. E' usato questo supporto poichè è molto più conveniente che gli hard disk ed è molto capiente. Il rovescio della medaglia è che i dati vengono copiati sequenzialmente,  nello stesso modo di una cassetta audio; Questo rende difficile la ricerca dei file da recuperare..(chi di noi non ha rotto un tasto RWD dello stereo!). Esistono vari tipi di backup.
Molti di questi lavorano con un flag sul file chiamato ARCHIVE BIT. Questo bit viene impostato a 1 quando il file viene creato o modificato così che al prossimo backup sarà copiato, ignorando invece i file con un ARCHIVE BIT impostato a 0. Ovviamente le cassette create dovranno esser tenute in un luogo sicuro e a prova di incendio. (non in mezzo al mare :-) 
Le 5 operazioni tipiche di backup sono:
FULL BACKUP
INCREMENTAL BACKUP
DIFFERENTIAL BACKUP
COPY BACKUP
DAILY BACKUP
FULL BACKUP:
Tutti i fil su un disco vengono copiati sul supporto a nastro infischiandone dell'ARCHIVE BIT.
INCREMENTAL BACKUP:
Fa il backup di tutti i file che sono stati creati o modificati dall'ultimo backup effettuato. Ogni backup incrementale è ovviamente legato al backup totale che è stato eseguito e dei successivi backup incrementali. I ARCHIVE BIT sono impostati a 0 durante un incremental backup.
Per ripristinare una situazione è necessario avere "installato" il backup totale e i vari backup incrementali.
DIFFERENTIAL BACKUP:
Fa il backup di tutti i file che sono stati creati o modificati dall'ultimo backup totale. I ARCHIVE BIT non vengono resettaqti a 0.
Questo significa che ogni file cambiato o creato dal backup totale verrà ricopiato.
COPY BACKUP:
Fa il backup dei file selezionati dall'utente. I ARCHIVE BIT non vengono resettati. Per ripristinare una situazione è necessario avere
"installato" il backup totale, più l'ultimo backup differenziale.
DAILY BACKUP:
Fa il backup dei file modificati dal giorno del backup totale. I ARCHIVE BIT non vengono resettati.
L'altro metodo di backup come abbiamo detto è il fault tolerant disk.
Questo tipo di ridondanza di dispositivi è nominata RAID (Reduntant Array of Inexpansive Disks).
Esistono vari tipi di RAID da 0-5.
RAID 0: Deposita i dati in dischi multipli, non vi è parita e nemmeno ridondanza.
Aumenta solamente la velocità di accesso ai dati.
RAID 1: Disk Mirroring scrive due identiche partizioni su dischi separati creando un backup automatico.
Disk Duplexing usa due hard disk con due diversi controller in modo di ridurre anche il possibile guasto del controller.In pratica Disk Mirroring hA due hard disk con le stesse partizioni ma un solo controller card, Disk Duplexing ho due controller card e due hard disk.
RAID 2: Scrive i dati in mulipli hard disk con correzione degli errori assegnata ad un disco specifico.
Non è consigliato..
RAID 3: Deposita i dati un bit alla volta, uno dei dischi del RAID 3 è dedicato esclusivamente all'immagazzinamento di parity check byte per ogni strip scritto. E' un buon metodo ma molto costoso a causa dell'alta ridondanza.
RAID 4: Deposita i dati un settore alla volta e ha un disco paritario dedicato.
Non è una soluzione ottimale a causa del tempo che impiega a scrivere e del costo dovuto all'alta ridondanza.
RAID 5: Deposita i dati e la parita viene scritta su multipli dischi (almeno 3). Le partizioni sui dischi devono essere di egual misura!
In pratica se un disco viene a mancare grazie alla parità scritta sugli altri dischi è possibile ricostruire i dati perduti. Inoltre in WINNT vi è bisogno di un disco in più perchè il settore di boot non può essere collocato su un RAID. Un vantaggio del RAID 5 è che i dischi sono "hot swappable" ovvero li posso cambiare a caldo senza spegnere la macchina.


How do I remove a stripe set?
To remove a stripe set, complete the following steps:
1. From the Start menu, select Programs, Administrative Tools, then Disk Administrator.
2. Select the stripe set you want to delete.
3. From the Partition drop-down menu, select Delete.
4. Confirm to delete the partition.
Note: You’ll lose all data on the stripe set.

How do I recreate a broken stripe set?
When a stripe set member with parity fails, you don’t receive a warning, and everything continues to work. However, when you start Disk Administrator, you get an indication of the failure on the graphical view of the disk set. The event log will also show the failure. To recreate the stripe set, complete the following steps:
1. Replace the faulty disk, and start Windows NT.
2. From the Start menu, select Programs, Administrative Tools, then Disk Administrator.
3. Select the stripe set you want to repair and an area of unpartioned space on the new physical disk.
4. From the Fault Tolerance menu, select Regenerate.

How do I break a mirror set?
If you lose part of a fault-tolerant volume (e.g., by hardware failure), the OS will display the message A disk that is part of a fault-tolerant volume can no longer be accessed. The drive will still be usable, but you won’t have the mirroring capability. To break the mirror set, complete the following steps:
1. From the Start menu, select Programs, Administrative Tools, then Disk Administrator.
2. The OS will display a message that a disk is missing.
3. Click on the mirror, and select Break Mirror from the Fault Tolerance menu.
4. Confirm the action.

How do I repair a broken mirror set?
You need an area of unpartitioned space that is at least the size of the primary partition to repair a broken mirror set. To perform the repair, complete the following steps:
1. From the Start menu, select Programs, Administrative Tools, then Disk Administrator.
2. Click the working part of the mirror, hold down the Ctrl key, and select the area of unpartitioned space.
3. Select Establish Mirror from the Fault Tolerance menu.

How do I create a mirror set (RAID 1)?


To create a mirror set, first create the primary partition. You can then perform the following steps to create a mirror of it:
1. From the Start menu, select Programs, Administrative Tools, then Disk Administrator.
2. Click the existing primary partition, and hold down the Ctrl key.
3. Click an unpartitioned area of disk space.
4. From the Fault Tolerance menu, select Establish Mirror.
5. From the Partition menu, select Commit changes now to begin duplication.
6. Reboot after the duplication process is completed.

How do I break a mirror set (RAID 1) in Windows 2000?
Breaking a mirror set won’t result in data loss but will give you two volumes with duplicate data.
To break a RAID 1 set in Windows 2000, perform the following steps:
1. From the Start menu, select Programs, Administrative Tools, then the Computer Management Microsoft Management Console (MMC) snap-in.
2. Expand the Storage branch, and select Disk Management.
3. Right click the mirror volume you want to remove, and select Break Mirror from the context menu. (In this step, you can also select Delete Mirror to remove both volumes that make up the mirror, but you lose the data on it.)
4. To confirm your selection, click Yes.
5. Another dialog might warn you about possible data loss on the broken mirror. Click Yes to continue.
You will now have two volumes, so you might want to delete the unwanted mirror to avoid confusion.

How do I create a mirror set (RAID 1) in Windows 2000?


All members of a RAID 1 volume set must be on a dynamic disk. To convert a disk from basic to dynamic, see 'Q. How do I convert a basic disk to dynamic?'.
To create a RAID 1 set in Windows 2000, complete the following steps:
1. From the Start menu, select Programs, Administrative Tools, then the Computer Management Microsoft Management Console (MMC) snap-in.
2. Expand the Storage branch, and select Disk Management.
3. Right click the partition you want to mirror, and select Add Mirror from the context menu.
4. Select the disk that will host the mirror, and click Add Mirror.
Click here to view image
5. If you mirror the boot partition, a dialog box details the changes that the program will make to boot.ini to enable mirror booting. Click OK.
Win2K shows the mirror set in a regenerating mode.

How do I convert a basic disk to dynamic?
Windows 2000 introduces the idea of a dynamic disk needed for fault tolerant configurations. To convert perform the following:
1. Start Computer Manager
2. Expand Storage - Disk Management.
3. Right click on the disk and select 'Upgrade to Dynamic Disk'
4. Select the disks to upgrade and click OK
5. A summary will be displayed.
6. Click Upgrade
7. Click Yes to the confirmation
Converting Basic disks to Dynamic disks don't require reboots - however any volumes contained on them after the conversion will generate a popup that basically says a re-boot is necessary before the volumes can be used. I generally say - NO, do not reboot - until all the volumes are identified and all the popups go away, then perform a single re-boot.
When you upgrade from basic to dynamic any existing partitions become simple volumes. Any existing mirrored, striped or spanned volumes sets created with NT 4.0 become dynamic mirrored, striped or spanned volumes respectively.
If you get a message that says you are out of space then you may not have enough unallocated free space at the end of the disk for the private region database that Dynamic disks use to keep volume information. To be Dynamic it needs about 1 MB of this space, sometime the space is not visible to the user in the GUI but it is still there.
You may not have the space if the partition(s) on the disk take up the entire disk and were created with Setup, an earlier version of NT or another OS. If partitions are created within Windows 2000 the space is reserved, partitions created with Setup will reserve the space in a later release.
To undo this conversion run Dmunroot.exe which will revert boot and system partition back to basic but all other volumes will be destroyed. Alternatively you should backup any data on the disk you wish to preserve, then delete all partitions - that should activate the menu choice "Revert to Basic Disk", the entire disk HAS to be unallocated or free space. Dmunroot is an unsupported utility available from Microsoft.

How do I regenerate a RAID 5 set in Windows 2000?


In Windows 2000, if you replace one part of a RAID 5 set as a result of faulty hardware, the volume won’t lose any data because of the stored parity information. However, you must replace the broken disk to re-enable the RAID 5 set’s fault-tolerant ability.
After you replace the bad disk, complete the following steps:
1. From the Start menu, select Programs, Administrative Tools, then the Computer Management Microsoft Management Console (MMC) snap-in.
2. Expand the Storage branch, and select Disk Management.
3. Win2K still shows the removed disk as Missing.
4. Right click an element of the RAID 5 volume, and select Repair Volume… from the context menu.
5. From the list, select a disk to use as the bad disk’s replacement and click OK.
Click here to view image
6. Win2K will show the set as regenerating.
Your RAID 5 set is now fault tolerant again, but you need to remove the RAID5 partition from the missing disk.
If you had other partitions on the disk that you removed, you can right click the partitions and select Delete Volume… to remove them. You should now right click the Missing text and select Remove Disk from the context menu.
N. 2 FOTO
If you ever reuse the original disk, Win2K displays the disk as Foreign. To read this disk, see 'Q. How do I import a foreign volume in Windows 2000?'

How do I delete a RAID 5 set in Windows 2000?
When you delete a RAID 5 set in Windows 2000, you lose all the data that the set contains. Therefore, make sure you first back up your data.
To delete a RAID 5 set, perform the following steps:
1. From the Start menu, select Programs, Administrative Tools, then the Computer Management Microsoft Management Console (MMC) snap-in.
2. Expand the Storage branch, and select Disk Management.
3. Right click an element of the RAID 5 volume, and select Delete Volume… from the context menu.
4. Click Yes to the confirmation.
Win2K will now list the space that the RAID 5 volume used as unpartitioned.

How do I create a RAID 5 set in Windows 2000?
Windows 2000 introduces dynamic disks, and all members of a RAID volume set must be on a dynamic disk. To convert a disk from basic to dynamic, see 'Q. How do I convert a basic disk to dynamic?'.
To create a RAID 5 set, set perform the following steps:
1. From the Start menu, select Programs, Administrative Tools, then the Computer Management Microsoft Management Console (MMC) snap-in.
2. Expand the Storage branch, and select Disk Management.
3. Right click an area of unallocated space, and select Create Volume from the context menu.
Click here to view image
4. Click Next to the Create Volume Wizard.
5. Select a RAID 5 volume type, and click Next.
Click here to view image
6. In the left pane, select the disks you want to use (at least three in total) and click Add.
7. Select the size to use from each disk. The size must be equal for each disk, so the largest space you can use is the smallest free space on any disk. After you select the size, click Next. If you select 1000MB from each disk, the total size would be only 2000MB because parity information uses a third of the space.
8. Select a drive letter to use, and click Next.
9. Select the file system to use and the label. You might also select whether to enable file and folder compression. Click Next.
10. The program displays a summary screen. Click Finish.
Win2K shows the disk areas as RAID 5 and in a regenerating mode.
You might receive the message The operation did not complete because the partition/volume is not enabled. Please reboot the computer to enable the partition/volume from the Logical Disk Manager. Click OK to this message, but don’t reboot until the regeneration is complete and Win2K shows the volume as healthy. Otherwise, you’ll need to reformat the partition upon reboot completion.
You might still need to reformat the volume, which indicates that the program has a bug.

Come installare un sistema RAID
Scegliendo la modalità RAID 1 (chiamata anche "mirror", ovvero "specchio") il secondo disco diventa una copia di riserva del primo, aggiornata automaticamente in tempo reale.
RAID1 non porta vantaggi immediati, anzi rallenta il computer perchè ogni dato deve essere scritto due volte (la prima sul disco principale e la seconda sul disco mirror), ma ha il grande pregio che se il primo disco si guasta, i dati sono salvi perchè si trovano anche sul secondo disco. E' ovvio che se si imbarca un virus, questo verrà replicato sul secondo disco.
La modalità RAID0 (chiamata anche stripe) accoppia i dischi diversamente: selezionandola, Windows vedrà un solo disco con capacità pari alla somma dei due dischi di partenza e velocità leggermente superiore a quella di un disco singolo.
La modalità RAID è quindi un modo vantaggioso per mettere frutto la presenza del secondo disco, però ha alcuni difetti. Il primo è che nell'istante in cui si attiva il RAID nel setup del BIOS, si perdono i dati contenuti in entrambi i dischi, che il PC interpreta come vergini e non formattati. Bisogna perciò eseguire sempre il backup di tutti i dati e avere a portata di mano il disco con il sistema operativo e i driver di periferica prima di attivare il RAID.
Un altro limite del RAID è che richiede un driver software speciale, che generalmente non è incluso nel CD di installazione del sistema operativo. Di solito è su un dischetto inserito nella confezione del computer o della scheda madre, che è indispensabile per portare a termine la reinstallazione di Windows XP. Quando il setup di Windows invita a premere F6 per installare i driver del controller SCSI, inserire il floppy con i driver RAID nel computer (i controller S-ATA vengono visti come controller esterni/SCSI), premere il tasto F6 e eseguire la procedura. Se il dischetto non è disponibile, prima di installare il secondo disco e attivare il RAID si dovrà scaricare il software dal sito Web del costruttore del computer o di quello della scheda madre/controller). La formattazione dei dischi RAID non è standard.
Se vogliamo cambiare la scheda madre, o il controller dei dischi, bisogna prima eseguire un backup completo dei dati e dei programmi, perchè collegando i dischi ad un controller o ad una scheda differente appariranno come vuoti e non formattati. Questa è una beffa soprattutto per chi sceglie RAID 1 per la sicurezza dei dati, in quanto se si guasta la scheda madre e un modello identico non è disponibile, i dati vanno persi. L'ultimo difetto del RAID è che se i dischi non sono identici, il meno capiente o il più lento dei due prevale. Per esempio, se il nuovo disco è da 250 Gb mentre quello originale è da 160 Gb, ai fini del RAID è come se avessimo comperato un secondo hard disc da 160 Gb: la capacità aggiuntiva non è utilizzabile. Questo difetto è insormontabile nel caso della modalità RAID1, mentre il setup di certi BIOS gestisce una modalità RAID speciale che permette di usare dischi con capacità diversa: è indicata come modalità JBOD.

Articolo sul RAID: Vedi qui


TIPS

RAID: La soluzione S-ATA su controller PCI ha prestazioni leggermente inferiore a causa dei 133 mbps di transfer rate massimo consentito da questo stesso bus, per cui la velocità è identica a quella dei controller ATA133 (Dischi Eide) integrati sulla scheda madre.


Soluzioni RAID a confronto

Livello RAID Descrizione Numero di dischi Capacità effettiva Tolleranza ai guasti Prestazioni in Lettura Prestazioni in Scrittura
0 Suddivisione dei blocchi di dati su più dischi in parallelo, senza controllo degli n>=2 n Nessuna n volte n volte
1 Duplicazione integrale dei dati su più dischi n>=2 1/n n-1 n volte 1 volta
5 Suddivisione dei blocchi di dati su più dischi in parallelo con singolo sistema di parità distribuito n>=3 1-1/n 1 (n-1) volte (n-1) volte
6 Suddivisione dei blocchi di dati su più dischi in parallelo con doppio sistema di parità distribuito n>=4 1-2/n 2 (n-2) volte (n-2) volte
10 Duplicazione integrale dei singoli dischi con suddivisione dei blocchi di dati sui volumi in parallelo n>=4 2/n 1 per ramo n volte (n/spans) volte)
50 Suddivisione dei dati su più volumi in parallelo; ciascun volume utilizza un sistema di parità distribuito di tipo Raid 5 n>=6 (nR-1)/nR 1 per ramo (n-1) volte per il numero di rami (n-1) volte per il numero di rami
60 Suddivisione dei dati su più volumi in parallelo; ciascun volume utilizza un sistema di parità distribuito di tipo 6 n>=8 (nR-2)/nR 2 per ramo (n-2) volte per il numero di rami (n-2) volte per il numero di rami

In questa tabella tutti i dischi considerati sono identici tra loro
In caso contrario tutto il sistema RAID è realizzato in funzione della capacità del disco più piccolo