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RAID (redundant arrays of independent/inexpensive disks) offers data security, improved access times or greater storage capacity than a single disk alone (where multiple disks are treated as a single volume), or some combination of all three depending on the configuration:

RAID 0 employs block-level striping which improves access times by splitting files across two disks, but at the cost of redundancy, which is nil. If either drive fails, the entire array is worthless unless an alternative backup is available.

RAID 1 employs mirroring which automatically duplicates data across two disks. Read-access is the same as for RAID 0, but write access is no better than a single disk. However, should either disk fail, the array will continue to function until the faulty drive is replaced, at which point the array is automatically rebuilt.

RAID 2 employs bit-level striping with dedicated parity, similar to RAID 0 but where individual bits are split sequentially rather than splitting sequential blocks. The disks must be perfectly synchronised and a separate parity disk ensures the array can be rebuilt should either striped disk fail. This form of RAID is theoretical and is not used in practice.

RAID 3 employs byte-level striping with dedicated parity similar to RAID 2 (with bytes rather than bits) but while it is not merely theoretical it is not commonly used either.

RAID 4 employs block-level striping with dedicated parity and requires at least three disks. However, the dedicated parity drive can cause bottlenecks which reduce overall performance.

RAID 5 employs block-level striping with distributed parity using three disks. However, performance is reduced upon failure of any one disk, and older configurations are at risk from the so-called RAID 5 write hole, which is potentially disastrous.

RAID 6 employs block-level striping with double distributed parity. The array can continue functioning with up to two drive failures, albeit at reduced performance.

RAID 1+0 (or RAID 10) employs both mirroring and block-level striping and requires a minimum of four disks. Multiple faults on a single mirror can be recovered from, but if any one disk from both mirrors fail, the entire array is lost.

RAID 0+1 is similar to RAID 1+0, but if an entire mirror fails, the entire array is lost.

RAID-5+3 (or RAID 53) employs both mirroring and block-level striping along with with distributed parity.

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