Configuring Software Raid - Red Hat ENTERPRISE LINUX 5 - DEPLOYMENT Deployment Manual

• Level 4 — Level 4 uses parity (see Note) concentrated on a single disk drive to protect data. It is
better 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 technologies 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 Enterprise Linux RAID installations. 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.
Note
RAID level 4 takes up the same amount of space as RAID level 5, but level 5 has more
advantages. For this reason, level 4 is not supported.
Note
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 reconstructed 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.
• 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.

4.5. Configuring Software RAID

Users can configure Software RAID during the graphical installation process (Disk Druid), the text-
based installation process, or during a kickstart installation.This chapter covers Software RAID
configuration during the installation process using the Disk Druid application.
• Apply software RAID partitions to the physical hard drives.
To add a boot partition (/boot/) to a RAID partition, ensure it is on a RAID1 partiton.
• Creating RAID devices from the software RAID partitions.
• Optional: Configuring LVM from the RAID devices.
Configuring Software RAID
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