Installation guide

Chapter 4. Redundant Array of Independent Disks (RAID) 65
high cost.
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The storage capacity of the level 1 array is equal to the capacity of one of the
mirrored hard disks in a Hardware RAID or one of the mirrored partitions in a Software
RAID.
• Level 4 — Level 4 uses parity
3
concentrated on a single disk drive to protect data. It is bet-
ter suited to transaction I/O rather than large file transfers. Because the dedicated parity
disk represents an inherent bottleneck, level 4 is seldom used without accompanying tech-
nologies such as write-back caching. Although RAID level 4 is an option in some RAID
partitioning schemes, it is not an option allowed in Red Hat Linux RAID installations
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The storage capacity of Hardware 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 mod-
ern CPUs and Software RAID, that usually isn’t 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.
2. RAID level 1 comes at a high cost because you write the same information to all of the disks in
the array, which wastes drive space. For example, if you have RAID level 1 set up so that your root (/)
partition exists on two 40G drives, you have 80G total but are only able to access 40G of that 80G. The
other 40G acts like a mirror of the first 40G.
3. Parity information is calculated based on the contents of the rest of the member disks in the array.
This information can then be used to reconstruct data when one disk in the array fails. The recon-
structed data can then be used to satisfy I/O requests to the failed disk before it is replaced and to
repopulate the failed disk after it has been replaced.
4. RAID level 4 takes up the same amount of space as RAID level 5, but level 5 has more advantages
than level 4. For this reason, level 4 is not supported.