VERITAS Volume Manager 3.5 Installation Guide (September 2004)

Getting Started with VxVM

System Setup Guidelines

Chapter 5

Dirty Region Logging (DRL) Guidelines

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.

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 with an even number of 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.