Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

500 Configuring Veritas Volume Manager
Guidelines for configuring storage
■ 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.
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.
Note: Using Dirty Region Logging can impact system performance in a write-intensive
environment.
For more information, see “Dirty region logging” on page 60.