Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

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.
■ 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.
See “Mirroring (RAID-1)” on page 41.
Dirty region logging guidelines
Dirty region logging (DRL) can speed up recovery of mirrored volumes following
a system crash. When DRL is enabled, Veritas Volume Manager keeps track of the
regions within a volume that have changed as a result of writes to a plex.
Warning: Using Dirty Region Logging can adversely impact system performance
in a write-intensive environment.
See “Dirty region logging” on page 57.
Striping guidelines
Refer to the following guidelines when using striping.
■ Do not place more than one column of a striped plex on the same physical disk.
Configuring Veritas Volume Manager
Guidelines for configuring storage
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