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514 Configuring Veritas Volume Manager
Guidelines for configuring storage
■ Leave the Veritas Volume Manager hot-relocation feature enabled. See “Hot-
relocation guidelines” on page 516 for details.
Mirroring guidelines
Refer to the following guidelines when using mirroring.
■ Do not place subdisks from different plexes of a mirrored volume on the
same physical disk. This action compromises the availability benefits of
mirroring and degrades performance. Using the
vxassist or vxdiskadm
commands precludes this from happening.
■ To provide optimum performance improvements through the use of
mirroring, at least 70 percent of physical I/O operations should be read
operations. A higher percentage of read operations results in even better
performance. Mirroring may not provide a performance increase or may
even result in a performance decrease in a write-intensive workload
environment.
Note: The operating system implements a file system cache. Read requests can
frequently be satisfied from the cache. This can cause the read/write ratio for
physical I/O operations through the file system to be biased toward writing
(when compared to the read/write ratio at the application level).
■ Where possible, use disks attached to different controllers when mirroring
or striping. Most disk controllers support overlapped seeks. This allows
seeks to begin on two disks at once. Do not configure two plexes of the same
volume on disks that are attached to a controller that does not support
overlapped seeks. This is important for older controllers or SCSI disks that
do not cache on the drive. It is less important for modern SCSI disks and
controllers. Mirroring across controllers allows the system to survive a
failure of one of the controllers. Another controller can continue to provide
data from a mirror.
■ A plex exhibits superior performance when striped or concatenated across
multiple disks, or when located on a much faster device. Set the read policy
to prefer the faster plex. By default, a volume with one striped plex is
configured to prefer reading from the striped plex.
For more information, see “Mirroring (RAID-1)” on page 42.