Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Dirty region logging in cluster environments
Dirty region logging (DRL) is an optional property of a volume that provides speedy
recovery of mirrored volumes after a system failure. DRL is supported in
cluster-shareable disk groups. This section provides a brief overview of how DRL
behaves in a cluster environment.
In a cluster environment, the VxVM implementation of DRL differs slightly from
the normal implementation.
A dirty region log on a system without cluster support has a recovery map and a
single active map. A CVM DRL, however, has a single recovery map per cluster
and one active map per cluster node.
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 for the size of the volume and the number of nodes.
It is possible to reimport 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 as a private disk group on 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
must allocate a new log of sufficient size.
See “Dirty region logging” on page 57.
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
Administering cluster functionality (CVM)
Dirty region logging in cluster environments
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