December 2002
Intermediate to advanced
260 pages
9h 18m
English
Content preview from Managing RAID on Linux
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RAID-5
RAID-5 eliminates the use of a dedicated parity drive and stripes parity information across each disk in the array, using the same XOR algorithm found in RAID-4 (see Figure 2-9). During each write operation, one chunk worth of data in each stripe is used to store parity. The disk that stores parity alternates with each stripe, until each disk has one chunk worth of parity information. The process then repeats, beginning with the first disk.
Figure 2-9. RAID-5 eliminates the dedicated parity disk by distributing
parity across all drives.
Take the example of a RAID-5 with five member disks. In this case, every fifth chunk-sized block on each member disk will contain parity information for the other four disks. This means that, as in RAID-1 and RAID-4, a portion of your total storage space will be unusable. In an array with five disks, a single disk’s worth of space is occupied by parity information, although the parity information is spread across every disk in the array. In general, if you have N disk drives in a RAID-5, each of size S, you will be left with (N-1) * S space available. So, RAID-4 and RAID-5 yield the same usable storage. Unfortunately, also like RAID-4, a RAID-5 can withstand only a single disk failure. If more than one drive fails, all data on the array is lost.
RAID-5 performs almost as well as a striped array for reads. Write performance on full stripe operations ...
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