RAID 5

RAID (redundant array of independent disks, originally redundant array of inexpensive disks) is a storage technology that combines multiple components of a disk drive into a logical unit. Data is distributed across the drives in one of several ways called "RAID levels", depending on the level of redundancy and performance required. The standard RAID levels are a basic set of RAID configurations that employ the techniques of stripping, mirroring or parity to create large reliable data stores from general purpose hard drives. The most common types today are RAID 0 (striping), RAID 1 and variants (mirroring), RAID 5 (distributed parity) and RAID 6 (dual parity).Upon drive failure while using RAID 5, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. RAID 5 will distribute parities evenly between all drives. Distributed parity provides a slight increase in performance. Real RAID 5 has the most common stripe size of 64k (65536 * 8 = 524288 bits). So when adding one drive for parity you will be able to rebuild the missing data in case of any drive failure.

RAID levels:

A number of standard schemes have evolved which are referred to as levels. There were five RAID levels originally, but many more variations have evolved.

RAID 0 (block-level stripping without parity or mirroring): It has no (or zero) redundancy. It provides improved performance and additional storage but no fault tolerance. Any drive failure destroys the array, and the likelihood of failure increases with more drives in the array.

RAID 1 (mirroring without parity or striping): In this level, data is written identically to two drives, thereby producing a "mirrored set"; the read request is serviced by either of the two drives containing the requested data, whichever one involves least seek time plus rotational latency. Similarly, a write request updates the stripes of both drives. The write performance depends on the slower of the two writes. At least two drives are required to constitute such an array.

RAID 2 (bit-level striping with dedicated Hamming-code parity): Here, all disk spindle rotation is synchronized, and data is striped such that each sequential bit is on a different drive. Hamming code parity is calculated across corresponding bits and stored on at least one parity drive.

RAID 3 (byte-level striping with dedicated parity): In this level, all disk spindle rotation is synchronized, and data are striped so that each sequential byte is on a different drive. Parity is calculated across corresponding bytes and stored on a dedicated parity drive.

RAID 4(block-level striping with dedicated parity): RAID 4 is equivalent to RAID 5 except that all parity data are stored on a single drive. Each drive operates independently, allowing I/O requests to be performed in parallel.

RAID 5 (block-level striping with distributed parity) distributes parity along with the data and requires all drives but one to be present to operate; the array is not destroyed by a single drive failure. Upon drive failure, any subsequent reads can be calculated from the distributed parity such that the drive failure is masked from the end user. RAID 5 requires at least three disks.

RAID 6 (block-level striping with double distributed parity) provides fault tolerance up to two failed drives. This makes larger RAID groups more practical, especially for high-availability systems. Like RAID 5, a single drive failure results in reduced performance of the entire array until the failed drive has been replaced and the associated data rebuilt.

In RAID 10, often referred to as RAID 1+0 (mirroring and striping), data is written in stripes across primary disks that have been mirrored to the secondary disks.

Refer this link, in case you need to know how to recover data from RAID 5 array after any kind of data loss situation.

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