VERITAS Volume Manager 3.1 Administrator's Guide

Initialization and Setup
System Setup Guidelines
Chapter 2 77
Follow these general guidelines when using mirroring. See “Mirroring
(RAID-1)” for more information.
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. Use of the vxassist
command precludes this from happening.
To provide optimum performance improvements through the use of
mirroring, at least 70 percent of the physical I/O operations should be
read operations. A higher percentage of read operations results in a
higher benefit of performance. Mirroring may not provide a
performance increase or result in performance decrease in a
write-intensive workload environment.
NOTE The UNIX 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 feasible, use disks attached to different controllers when
mirroring or striping. Most disk controllers support overlapped seeks
that allow seeks to begin on two disks at once. Do not configure two
plexes of the same volume on disks 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
many newer SCSI disks and controllers used in most modern
workstations and server machines. Mirroring across controllers can
be of benefit because the system can survive a controller failure. The
other controller can continue to provide data from the other mirror.
A plex can exhibit superior performance due to being striped or
concatenated across multiple disks, or because it is located on a much
faster device. The read policy can then be set to prefer the “faster”
plex. By default, a volume with one striped plex is configured with
preferred reading of the striped plex.