VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Chapter 10, Administering Cluster Functionality
Dirty Region Logging (DRL) in Cluster Environments
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Dirty Region Logging (DRL) 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 DRL and
describes how DRL behaves in a cluster environment. For more information on DRL, see
“Dirty Region Logging (DRL)” on page 40.
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 accommodatea recoverymap plus active mapsfor each nodein the cluster. Thesize
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
groupina clusterenvironment.However, the dirty regionlogs of theimporteddisk 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.