VERITAS Volume Manager 3.1 Administrator's Guide

Cluster Functionality
Dirty Region Logging and Cluster Environments
Chapter 7308
Dirty Region Logging and Cluster
Environments
Dirty Region Logging (DRL) is an optional property of a volume that
provides speedy recovery of mirrored volumes after a system failure.
Dirty Region Logging is supported in cluster-shareable disk groups. This
section provides a brief overview of DRL and describes how DRL behaves
in a clusterenvironment. For more information on DRL, see “Dirty
Region Logging (DRL) Guidelines” and “Dirty Region Logging”.
DRL keeps track of the regions that have changed due to I/O writes to a
mirrored volume and uses thisinformation to recover only the portions of
the volume that need to be recovered. DRL logically divides a volume
into a set of consecutive regions and maintains a dirty region log that
contains a status bit representing each region of the volume. Log
subdisks are used to store the dirty region log of a volume that has DRL
enabled. A volume with DRL has at least one log subdisk, which is
associated with one of the volume plexes.
Before writing any data to the volume, the regions being written are
marked dirty in the log. If a write causes a log region to become dirty
when it was previously clean, the log is synchronously written to disk
before the write operation can occur. A log region becomes clean again
after the write to the mirror is complete. On system restart, the Volume
Manager recovers only those regions of the volume which are marked as
dirty in the dirty region log.
In a cluster environment, the Volume Manager 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.
Log Format and Size
As with VxVM in the non-clustered case, the clustered dirty region log
exists on a log subdisk in a mirrored volume.
A VxVM dirty region log has a recovery map and a single active map. A
clustered dirty region log, however, has one recovery map and multiple
active maps (one for each node in the cluster). Unlike VxVM, the cluster
feature places the recovery map at the beginning of the log.