VERITAS Volume Manager 3.1 Administrator's Guide

Initialization and Setup
System Setup Guidelines
Chapter 278
Dirty Region Logging (DRL) Guidelines
NOTE You must license the VERITAS Volume Manager product to use this
feature.
Dirty Region Logging (DRL) can speed up recovery of mirrored volumes
following a system crash. When DRL is enabled, Volume Manager keeps
track of the regions within a volume that have changed as a result of
writes to a plex. 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.
Follow these guidelines when using DRL:
• For Dirty Region Logging 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.
• It is possible to “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 with two or more sectors (preferably
an even number, as the last sector in a log subdisk with an odd
number of sectors is not used). The log subdisk size is normally
proportional to the volume size. If a volume is less than 2 gigabytes, a
log subdisk of 2 sectors is sufficient. The log subdisk size should then
increase by 2 sectors for every additional 2 gigabytes of volume size.
However, the vxassist command chooses reasonable sizes by
default. In general, use of the default log subdisk length provided by