VERITAS Volume Manager 3.1 Administrator's Guide

Cluster Functionality
Dirty Region Logging and Cluster Environments
Chapter 7 309
The clustered dirty region log size is typically larger than a VxVM dirty
region log, as it must accommodate active maps for all nodes in the
cluster plus a recovery map. The size of each map within the dirty region
log is one or more whole blocks. vxassist automatically takes care of
allocating 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 should be one block long for each two
gigabytes of volume size. For a two-gigabyte volume in a two-node
cluster, a log size of three blocks (one block per map) should be sufficient;
this is the minimum log size. A four-gigabyte volume in a four-node
cluster requires a log size of ten blocks, and so on.
When nodes are added to an existing cluster, the existing DRL logs need
to be detached and removed (using the vxplex -o rm dis command) and
then recreated (using the vxassist addlog command). The use of these
two commands increases the log sizes so that they can accommodate
maps for the additional nodes.
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.
It is possible to import a VxVM disk group (and its volumes) as a shared
disk group in a cluster environment and vice versa. However, the dirty
region logs of the imported disk group may be considered invalid and a
full recovery may result.
If a shared disk group is imported by a VxVM system without cluster
support, VxVM considers the logsof the shared volumes to be invalid and
conducts a full volume recovery. After this recovery completes, the
Volume Manager uses the cluster feature’s Dirty Region Logging.
The Volume Manager cluster feature is capable of performing a DRL
recovery on a non-shared VxVM volume. However, if a VxVM volume is
moved to a VxVM system with cluster support and imported as shared,
the dirty region log is probably too small to accommodate all the nodes in
the cluster. The cluster feature therefore marks the log invalid and
performs a full recovery anyway. Similarly, moving a DRL volume from a
two-node cluster to a four-node cluster can result in too small a log size,
which the cluster feature handles with a full volume recovery. In both
cases, the system administrator is responsible for allocating a new log of