Red Hat ENTERPRISE LINUX 4 System Administration Manual page 109

option allowed in Red Hat Enterprise Linux RAID installations.
RAID level 4 is equal to the capacity of member disks, minus the capacity of one member disk. The
storage capacity of Software RAID level 4 is equal to the capacity of the member partitions, minus
the size of one of the partitions if they are of equal size.
• Level 5 — This is the most common type of RAID. By distributing parity across some or all of an
array's member disk drives, RAID level 5 eliminates the write bottleneck inherent in level 4. The only
performance bottleneck is the parity calculation process. With modern CPUs and Software RAID,
that usually is not a very big problem. As with level 4, the result is asymmetrical performance, with
reads substantially outperforming writes. Level 5 is often used with write-back caching to reduce
the asymmetry. The storage capacity of Hardware RAID level 5 is equal to the capacity of member
disks, minus the capacity of one member disk. The storage capacity of Software RAID level 5 is
equal to the capacity of the member partitions, minus the size of one of the partitions if they are of
equal size.
• Linear RAID — Linear RAID is a simple grouping of drives to create a larger virtual drive. In linear
RAID, the chunks are allocated sequentially from one member drive, going to the next drive only
when the first is completely filled. This grouping provides no performance benefit, as it is unlikely
that any I/O operations will be split between member drives. Linear RAID also offers no redundancy
and, in fact, decreases reliability — if any one member drive fails, the entire array cannot be used.
The capacity is the total of all member disks.
RAID Levels and Linear Support
4
The storage capacity of Hardware
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