VERITAS Volume Manager 3.5 Installation Guide (August 2002)

System Setup Guidelines
64 VERITAS Volume Manager Installation Guide
Dirty Region Logging (DRL) Guidelines
Dirty Region Logging (DRL) can speed up recovery of mirrored volumes following a
systemcrash. WhenDRL isenabled, VERITAS Volume Manager keepstrackof the regions
within a volume that have changed as a result of writes to a plex. VERITAS Volume
Manager maintains a bitmap and stores this information in a log subdisk. Log subdisks
are defined for and added to a volume to provide DRL. Log subdisks are independent of
plexes, are ignored by plex policies, and are only used to hold the DRL information.
Note Using Dirty Region Logging can impact system performance in a write-intensive
environment.
Refer to the following guidelines when using DRL:
◆ For DRL to be in effect, the volume must be mirrored.
◆ At least one log subdisk must exist on the volume for DRL to work. However, only
one log subdisk can exist per plex.
◆ The subdisk that is used as the log subdisk should not contain necessary data.
◆ Mirror log subdisks by having more than one log subdisk (but only one per plex) in
the volume. This ensures that logging can continue, even if a disk failure causes one
log subdisk to become inaccessible.
◆ Log subdisks must be configured withan even numberof sectors. (The last sector in a
log subdisk with an odd number of sectors is not used.) The log subdisk size should
usually be configured as 33 sectors per 2 GB of volume. A log subdisk of 33 sectors is
sufficient for a volume larger than 2 GB. In general, the default log subdisk length
configured by the vxassist command is suitable for use.
◆ Do not place the log subdisk on a heavily-used disk, if possible.
◆ Use persistent (non-volatile) storage disks for log subdisks.
Mirroring and Striping Guidelines
Note The VxVM Full license is required to use this feature.
Refer to the following guidelines when using mirroring and striping together. For more
information, see the VERITAS Volume Manager Administrator’s Guide for details.
◆ Make sure enough disks are available for the striped and mirrored configuration. At
least two disks are required for the striped plex and one or more additional disks are
needed for the mirror.
◆ Never place subdisks from one plex on the same physical disk as subdisks from the
other plex.