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

■ Calculate stripe-unit sizes carefully. In general, a moderate stripe-unit size
(for example, 64 kilobytes, which is also the default used by vxassist) is
recommended.
■ If it is not feasible to set the stripe-unit size to the track size, and you do not
know the application I/O pattern, use the default stripe-unit size.
■ Many modern disk drives have variable geometry. This means that the track
size differs between cylinders, so that outer disk tracks have more sectors than
inner tracks. It is therefore not always appropriate to use the track size as the
stripe-unit size. For these drives, use a moderate stripe-unit size (such as 64
kilobytes), unless you know the I/O pattern of the application.
■ Volumes with small stripe-unit sizes can exhibit poor sequential I/O latency
if the disks do not have synchronized spindles. Generally, striping over disks
without synchronized spindles yields better performance when used with
larger stripe-unit sizes and multi-threaded, or largely asynchronous, random
I/O streams.
■ Typically, the greater the number of physical disks in the stripe, the greater
the improvement in I/O performance; however, this reduces the effective mean
time between failures of the volume. If this is an issue, combine striping with
mirroring to combine high-performance with improved reliability.
■ If only one plex of a mirrored volume is striped, set the policy of the volume
to prefer for the striped plex. (The default read policy, select, does this
automatically.)
■ If more than one plex of a mirrored volume is striped, configure the same
stripe-unit size for each striped plex.
■ Where possible, distribute the subdisks of a striped volume across drives
connected to different controllers and buses.
■ Avoid the use of controllers that do not support overlapped seeks. (Such
controllers are rare.)
The vxassist command automatically applies and enforces many of these rules
when it allocates space for striped plexes in a volume.
See “Striping (RAID-0)” on page 38.
RAID-5 guidelines
Refer to the following guidelines when using RAID-5.
In general, the guidelines for mirroring and striping together also apply to RAID-5.
The following guidelines should also be observed with RAID-5:
555Configuring Veritas Volume Manager
Guidelines for configuring storage