RAID Data Recovery
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The RAID concept was first articulated by a group of Berkeley re-searchers in 1987. Their work defined five levels of RAID: five ways of distributing data across an array of discs so that the failure of a single disc does not cause data loss. Of the original designs, three have proven commercially attractive: Level 1, Level 3 and Level 5. Though not strictly RAID, an additional level, Level 0, is commonly included among RAID designs because of its similarity to the original designs. Level 0 distributes data across discs by striping-a technique for storing consecutive chunks of data on different discs. Striping improves I/O throughput but provides no more data protection than conventional drives. Level 1 achieves data redundancy through mirroring-keeping identical data on two separate discs. Mirroring offers excellent reliability and some improvement in I/O throughput but is relatively inefficient in its use of total disc capacity. Level 3 and Level 5 combine striping with parity codes to allow recovery of data if a disc fails. Level 3 and Level 5 both use disc capacity more efficiently than Level 1.
Most Common RAID Configurations:
RAID 0:
RAID 0 / known as data striping, distributes data across multiple drives which results in higher data throughput. Striped volumes are composed of stripes of data of equal size written across each disk in the volume. They are created from equally sized, unallocated areas on up to 32 physical disks.
Striped volumes do not contain redundant information. If one disk fails, the whole striped volume fails and no data can be recovered. The reliability for the striped volume is less than the least reliable disk in the set. Stripe sets are used for performance reasons. In general, striped volumes work well when you need to distribute I/O operations. Access to the data on a striped volume is usually faster than access to the same data would be on a single disk, because the I/O is spread across more than one disk.
Advantages of RAID 0:
The reason RAID 0 is a performance-enhancing configuration is that striping enables the array to directly access data from the multiple drives simultaneously. As the data is spread across a number of drives in the array, it can be accessed faster without having "bottle neck" effect on read/write access, as with a single drive. This is highly advantageous when retrieving large files, as the file data is spread out effectively across multiple drives with direct accessibility yet treated as fragments on the data stripes.
Disadvantage of RAID 0:
Striping is optimized for performance at the expense of fault tolerance (no space made available to store the redundant data. Any drive failure in the RAID 0, data loss is imminent).
RAID 1:
The RAID 1 or mirrored set has enhanced reliability achieved by providing an identical twin of the selected volume. All data written to the mirrored volume is written to both volumes, so no data is lost if there is a drive failure. If there is a read failure on one of the disks, the data is read from the other disk in the mirrored volume. If there is a write failure on one of the disks in the mirrored volume, the remaining disk is used for all subsequent accesses. This provides a full level of redundancy for the data on the system as the data is mirrored on to both drives. If one of the drives fails, the other drive still has all the data that existed in the system.
Advantages of RAID 1:
Random disk read operations on mirrored volumes are more efficient than on a single volume. The ability to load balance read operations across the physical disks. With current SCSI technology, two disk read operations can be done simultaneously.
Recovery from a disk failure is very rapid. Mirrored volumes offer the fastest data recovery, with the least impact on system performance, because the shadow volume contains all of the data. There is no data re-computation needed to restore the system.
Disadvantages of RAID 1
Disk write operations are less efficient. Because data must be written to both disks, there is a slight performance penalty.
Mirrored volumes are the least efficient in terms of space utilization.
RAID 5:
A RAID-5 volume dedicates the equivalent of the space of one disk in the RAID-5 volume for storing the parity stripes, but distributes the parity stripes across all the disks in the group. The data and parity information are arranged on the volume so that they are always on different disks.
Implementing a RAID-5 volume requires a minimum of three and a maximum of 32 disks in the set. The physical disks do not need to be identical. However, there must be equal size blocks of unused space available on each physical disk in the set. The disks can be on the same or different controllers.
If one of the disks in a RAID-5 volume fails, none of the data is lost. When a read operation requires data from the failed disk, the system reads all of the remaining good data stripes in the stripe and the parity stripe. Each data stripe is subtracted (with XOR) from the parity stripe; the order is not important. The result is the missing data stripe.
Advantages of RAID 5
RAID-5 volumes are well suited for large sequential reads. Typically, RAID-5 is used in database mining.
Disadvantages of RAID-5
In general, RAID-5 volumes are not well suited for any write intensive workload, since a single write is likely to generate a disk read of the parity and two writes (to update data and to parity).
RAID 0/1 or 10:
Raid 0/1 or 10 - is a dual level array that utilizes RAID 0 and RAID1 (mirrored) sets into a single array. Data is striped across all mirrored sets.
As a comparison to RAID 5 where lower cost and fault tolerance is important, RAID 0/1 utilizes several drives, in order to provide better performance. Each drive in the array is duplicated (mirrored). This eliminates the overhead and delay of parity. This level array offers high data transfer advantages of striped arrays and increased data accessibility (reads). System performance during a drive rebuild is also better than that of parity based arrays, since data does not need to be regenerated from parity information, but copied from the other mirrored drive.
RAID 0/5 or 50:
RAID 0/5 and 50 form large arrays by combining the block striping and parity of RAID 5 with the straight block striping of RAID 0.
RAID 0/5 is a RAID 5 array comprised of a number of striped RAID 0 arrays; it is less commonly seen than RAID 50, which is a RAID 0 array striped across RAID 5 elements.
RAID 50 and 05 improve upon the performance of RAID 5 through the addition of RAID 0, particularly during writes. It also provides better fault tolerance than the single RAID level does, especially if configured as RAID 50.
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