Raid 5 (Parity); Raid 6 (Dual Parity) - Silicon Graphics Rackable C2005 2U User Manual

RAID 5 (Parity)

RAID 6 (Dual Parity)

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RAID 1 has administrative advantages: it allows 24/7 environments to do backups on the array.
Mirroring is useful when critical data needs to be stored and backed up.
A RAID 5 array uses block-level striping with parity data distributed across all the member disks.
RAID 5 is the most popular RAID level and is used in both hardware and software RAID. You
can create a RAID 5 array using disks of different sizes, but the storage space added to the array
by each disk will be limited to the size of the smallest disk in the array.
Example: If a 120GB disk is used to build a RAID 5 array with two 100GB disks, each disk
will donate 100GB to the array. 200GB will be used for storage while 100GB will be used
for parity information.
Every time a data block is written on a disk, a parity block is generated within the same stripe. If
another block is written on that same stripe, the parity block is recalculated and rewritten. The disk
used for the parity block is staggered from one stripe to the next.
The parity blocks are not read on data reads. The parity blocks are used when a read of a data sector
results in a CRC (cyclic redundancy check) error. In a case like this, the sector in the same relative
position within each of the remaining data blocks in the stripe and within the parity block are used
to reconstruct the sector with the error. Also, if a disk were to fail in a RAID 5 array, the parity
blocks from the surviving disks are combined mathematically with the data blocks from the
surviving disks to reconstruct the data on the failed drive.
The maximum number of drives in a RAID 5 array are theoretically unlimited. However, with a
greater number of disks in an array, there is a greater chance of drives failing in succession.
Striped set with dual distributed parity. Provides fault tolerance from two drive failures; the array
continues to operate with up to two failed drives. With dual parity, it gives time to rebuild the array
without the data being volatile while the failed drive is being recovered.
RAID 5 (Parity)
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