RAID (Redundant Array of Independent Disks)
Technology
Technology
l A natural solution is a large array of small
independent disks acting as a single higher-performance logical disk. A concept
called data striping is used,
which utilizes parallelism to improve
disk performance.
l Data striping distributes data transparently over
multiple disks to make them appear as a single large, fast disk.
Different raid organizations were defined based on different
combinations of the two factors of granularity of data
interleaving (striping)
and pattern used to compute redundant
information.
l Raid
level 0 has no redundant data and hence has the best
write performance.
l Raid level 1 uses mirrored disks.
l Raid level 2 uses memory-style redundancy by using
Hamming codes, which contain parity bits for distinct
overlapping subsets of
components. Level 2 includes both
error detection and correction.
l Raid level 3 uses a single
parity disk relying on the disk
controller to figure out which disk has failed.
l Raid Levels 4 and 5 use
block-level data striping, with
level 5 distributing data and parity
information across all
disks.
l Raid level 6 applies the
so-called P + Q redundancy
scheme using Reed-Soloman codes to protect against up
to
two disk failures by using just two redundant disks.
Use of RAID Technology
Different raid organizations are being used under
different
situations
l Raid
level 1 (mirrored disks)is the easiest for rebuild of a disk from other disks– It is used for critical applications like l
Raid level 2 uses memory-style redundancy by using
Hamming codes, which contain parity bits for distinct overlapping subsets of
components. Level 2 includes both error detection and correction.
l Raid level 3 ( single parity
disks relying on the disk controller to figure out which disk has failed) and
level 5 (block-level data striping) are preferred for Large volume storage,
with level 3 giving higher transfer rates.
l Most popular uses of the RAID
technology currently are: Level 0 (with striping), Level 1 (with mirroring) and
Level 5 with an extra drive for parity.
l Design Decisions for RAID
include – level of RAID, number of disks, choice of parity schemes, and
grouping of disks for block-level striping.
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