VERITAS Volume Manager 4.1 Administrator's Guide

Administering Cluster Functionality
Dirty Region Logging (DRL) in Cluster Environments
Chapter 10 369
In a cluster environment, the VxVM implementation of DRL differs
slightly from the normal implementation. The following sections outline
some of the differences and discuss some aspects of the cluster
environment implementation.
Header Compatibility
Except for the addition of a cluster-specific magic number, DRL headers
in a cluster environment are the same as their non-clustered
counterparts.
Dirty Region Log Format and Size Requirements
As in the non-clustered case, the dirty region log in clusters exists on a
log subdisk in a mirrored volume.
A dirty region log on a system without cluster support has a recovery
map and a single active map. A dirty region log in a cluster, however, has
one recovery map and one active map for each node in the cluster). The
cluster functionality of VxVM places the recovery map at the beginning
of the log.
The dirty region log size in clusters is typically larger than in
non-clustered systems, as it must accommodate a recovery map plus
active maps for each node in the cluster. The size of each map within the
dirty region log is one or more whole blocks. The vxassist command
automatically allocates a sufficiently large dirty region log.
The log size depends on the volume size and the number of nodes. The
log must be large enough to accommodate all maps (one map per node
plus a recovery map). Each map must be one block long for each 2
gigabytes of volume size. For a 2-gigabyte volume in a 2-node cluster, a
log size of 2 blocks (one block per map) is sufficient; this is the minimum
log size. A 4-gigabyte volume in a 4-node cluster requires a log size of 10
blocks, and so on.
It is possible to re-import a non-shared disk group (and its volumes) as a
shared disk group in a cluster environment. However, the dirty region
logs of the imported disk group may be considered invalid and a full
recovery may result.