Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

Appendix B, Configuring VERITAS Volume Manager
Guidelines for Configuring Storage
449
Note Using Dirty Region Logging can impact system performance in a write-intensive
environment.
For more information, see “Dirty Region Logging (DRL)” on page 42.
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.
◆ 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.
Note 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.