RAID. Contents. Definition and Use of the Different RAID Levels. The different RAID levels: Definition Cost / Efficiency Reliability Performance

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RAID

Definition and Use of the Different RAID Levels

What is RAID?

 RAID(Redundant Array of Independent Disks)

 was first defined by Patterson, Gibson, and Katz from the Berkeley University in 1987,

 uses logical volumes consisting of several disks.

 The RAID leveldefines how the disks are organized,

 Depending on the RAID level, the reliabilityand/or the performancewill be improved.

 RAID systems can be implemented  in software

Operating Systems 2 RAID 3 Chr. Vogt  in software

 in hardware

 RAID controllers

 external RAID subsystems

 RAID functionality offered by the storage arrays in a SAN (Storage Area Network)

RAID Levels

 Originally, five RAID levels were defined:  RAID 1: Mirrored disks

 RAID 2 H i d f ti

 RAID 2: Hamming code for error correction

 RAID 3: Single parity disk, bit-interleaved parity

 RAID 4: Single parity disk, block-level parity

 RAID 5: Data and parity spread over all disks (block-level distributed parity)

 Later, further RAID levels were defined:  RAID 0: Striped disks (no redundancy)

 RAID 0+1: a combination of RAID 0 and RAID 1 (also called RAID 10)

Operating Systems 2 RAID 4 Chr. Vogt  RAID 6: dual redundancy using a Reed-Solomon code

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Evaluation Criteria for the RAID Levels

The following aspects need to be considered when evaluating the different RAID levels:  Space efficiency / cost: How much additional disk space is required?

 Space efficiency / cost: How much additional disk space is required?  Reliability: How many disks may fail without losing data?

 Also: What reliability remains after a disk failure?

 Performance:

 during normal operation,

 after a disk failure,

 during the restoration of a failed disk.

Operating Systems 2 RAID 5 Chr. Vogt  during the restoration of a failed disk.

In many cases

 read and write operations have to be considered separately,

 the performance also depends on the size of the I/O.

 Restoration: What needs to be done in order to restore the data of a failed disk?

RAID 0 or Striping

For a RAID 0 array with n disks, data is divided into n strips which are written to the disks in a round-robin fashion.

Space efficiency / cost: No redundancy, 100% storage efficiency, no extra cost. Reliability: No redundancy, no reliability.

Performance: Very good, when the disks can work in parallel.

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RAID 1 or Mirroring

The same data is written to two or more disks.

Operating Systems 2 RAID 7 Chr. Vogt Space efficiency / cost: 1/n for an n-fold mirror / n times as many disks are needed Reliability: n-1 disks can fail without losing data

Performance: Read performance is improved: read from any disk (in parallel) Write performance is unchanged: parallel writes to all disks Restoration: Copy the data from a remaining disk in the mirror set

RAID 0+1 (1)

RAID 0+1 sometimes is also called RAID 10. It can be built in two different ways:

Mirrored Stripesets Striped Mirrorsets

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RAID 0+1 (2)

Reliability:

Mirrored Stripesets: Any disk can fail without losing data (2-fold mirroring). A failing disk makes the whole stripe set inaccessible A failing disk makes the whole stripe set inaccessible. Striped Mirrorsets: Any disk can fail without losing data (2-fold mirroring).

1 disk can fail per mirror set in the stripe set. Restoration(of a single failed disk):

Mirrored Stripesets: A whole stripeset needs to be copied from one of the remaining copies in the mirrorset.

Striped Mirrorsets: The data on the failed disk needs to be copied from one of the i i di k i th i t

Operating Systems 2 RAID 9 Chr. Vogt

remaining disks in the mirrorset. RAID 0+1

 should always be built as striped mirrorsets,

 is the best possible solution when cost is not the main concern.

RAID 3 (1)

For a RAID 3 array with n disks, data

 is divided into n-1 strips (typically small ones),

 parity information (bitwise XOR) is calculated over the n 1 strips and stored on the n th disk

 parity information (bitwise XOR) is calculated over the n-1 strips and stored on the n-th disk.

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RAID 3 (2)

Performance:

Normal operation:p

 Good read performance (disks are accessed in parallel).

 Good write performance for large I/O requests (more than n-1 strips), because all disks are accessed in parallel.

 Very bad performance for small writes which access only one or a few disks:

 read the old data from the disks, which will not be overwritten,

 calculate the new parity,

 write the new data and the new parity.

After a disk failure: Performance decreases, because the missing data must be restored from the old data and the parity information

Operating Systems 2 RAID 11 Chr. Vogt

restored from the old data and the parity information. In either situation, the single parity disk can become a bottleneck.

Restoration: In order to restore the contents of a failed disk, all remaining disks, including the parity disk, must be read. Hence, there will be a heavy I/O load on the disks during the restoration.

RAID 5 (1)

For a RAID 5 array with n disks, data

 is divided into n-1 strips (typically large ones),

 parity information (bitwise XOR) is calculated over the n 1 strips

 parity information (bitwise XOR) is calculated over the n-1 strips,

 the parity information is distributed over the disks.

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RAID 5 (2)

Performance:

Normal operation:

 Good read performance (disks are accessed in parallel)

 Good read performance (disks are accessed in parallel).

 Because of the big stripe size, I/Os typically only access one or a few disks.

 Very bad performance for small writes:

 read the old data that is to be changed and the old parity information,

 calculate the new parity (from the old data, the old parity, and the new data),

 write the new data and the new parity.

After a disk failure: Performance decreases, because the missing data must be restored from the old data and the parity information.

Operating Systems 2 RAID 13 Chr. Vogt Restoration: In order to restore the contents of a failed disk, all remaining disks,

containing data and parity, must be read. Hence, there will be a heavy I/O load on the disks during the restoration.

(Same as with RAID 3.)

Adaptive RAID 3+5

 Some vendors claim that their RAID systems (software or hardware) perform adaptive RAID 3 / RAID 5 ti

RAID 3 / RAID 5 operations.

 The placement of the parity information is fixed, and cannot be changed dynamically.  The adaptation lies in the choice of the write algorithm, depending on the I/O size:

 perform large I/Os by writing all data and the parity in parallel,

 perform small I/Os by using the algorithm described for RAID 5,

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RAID 6

For a RAID 6 array with n disks,  data is divided into n-2 strips,

 two pieces of parity information are calculated over the n-2 strips (using a Reed-Solomon code),

 two pieces of parity information are calculated over the n 2 strips (using a Reed Solomon code),

 the parity information is distributed over the disks.

Operating Systems 2 RAID 15 Chr. Vogt Space efficiency / cost: (n-2)/n when using n disks / 2 additional disks are needed Reliability: 2 disks may fail without losing data.

Performance and Restoration: Similar to RAID 5.

Further Aspects for High Availability

 RAID sets protect against the loss of data in case of a disk failure.

 I dditi i i d t b t k t

 In addition, provisions need to be taken to  make the access to the data highly available,

 improve the performance of I/O operations.

 In order to make data access highly available, a RAID system should provide  redundant power supplies, fans, etc.

 several busses for attaching the disks,

 redundant controllers

Operating Systems 2 RAID 16 Chr. Vogt  with load balancing for the I/O requests,

 with cache takeover in case of a controller failure,

 hot-swappable components (disks, power supplies, etc.),

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Performance Optimization Features

 Caches, which in particular can improve the bad write performance of RAID3 and RAID5 sets.  To avoid data inconsistencies („write hole“), caches must be protected with a battery backup.

 Load balancing between redundant controllers.  In mirrorsets,

 load balance the read requests between the disks in the mirrorset,

 read from the disk with the optimal head position.

 Since I/O performance often depends on the size of the I/Os, a RAID system should  gather statistical information about the I/O sizes,

 allow the stripe size (or: chunk size) to be adjusted for each RAID set individually

Operating Systems 2 RAID 17 Chr. Vogt  allow the stripe size (or: chunk size) to be adjusted for each RAID set individually.

 Since restoring a failed disk causes a heavy additional I/O load, it should be possible to decide whether to give preference to

 the restore operation, in order to restore redundancy as quickly as possible,