Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

404 Administering cluster functionality
Dirty region logging in cluster environments
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 dirty region log in a cluster, however, has one recovery map and one active
map for each node in the cluster.
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 60.
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/