VERITAS Volume Manager 3.5 Administrator's Guide (September 2004)

Administering Cluster Functionality
Dirty Region Logging (DRL) in Cluster Environments
Chapter 10358
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.
If a shared disk group is imported by a system without cluster support,
VxVM considers the logs of the shared volumes to be invalid and
conducts a full volume recovery. After the recovery completes, VxVM
uses DRL.
The cluster functionality of VxVM can perform a DRL recovery on a
non-shared volume. However, if such a volume is moved to a VxVM
system with cluster support and imported as shared, the dirty region log
is probably too small to accommodate maps for all the cluster nodes.
VxVM then 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 functionality
of VxVM handles with a full volume recovery. In both cases, you are
responsible for allocating a new log of sufficient size.
To increase the size of an existing DRL log so that it can accommodate
maps for extra nodes, use the vxplex -o rm dis command to detach and
remove the log plex, and then use the vxassist addlog command to
recreate the log.
How DRL Works in a Cluster Environment
When one or more nodes in a cluster crash, DRL must handle the
recovery of all volumes that were in use by those nodes when the crashes
occurred. On initial cluster startup, all active maps are incorporated into
the recovery map during the volume start operation.
Nodes that crash (that is, leave the cluster as dirty) are not allowed to
rejoin the cluster until their DRL active maps have been incorporated
into the recovery maps on all affected volumes. The recovery utilities
compare a crashed node’s active maps with the recovery map and make
any necessary updates before the node can rejoin the cluster and resume
I/O to the volume (which overwrites the active map). During this time,
other nodes can continue to perform I/O.