VERITAS Volume Manager 3.1 Administrator's Guide

Volume Manager Operations
FastResync (Fast Mirror Resynchronization)
Chapter 3 119
FMR and Writable Snapshots
One of two options is used to track changes to a writable snapshot, as
follows:
• create a separate map that tracks changes to a snapshot volume
• update the map of the parent of the snapshot volume. Use this
shortcut method only if there are few updates to the snapshot volume,
such as in the backup and DSS (decision support systems)
applications
For VxVM 3.1, the latter method is implemented; i.e., the map of the
parent of the snapshot volume is updated when writing to the snapshot.
Caveats and Limitations
FMR is not supported on RAID-5 volumes.
When a subdisk is relocated, the entire plex is marked “dirty” and a full
resynchronization becomes necessary.
FMR Enhancements to VxVM Snapshot Functionality
The FMR snapshot enhancements to Release 3.1 extend the snapshot
model as shown in the diagram below. Beginning with Release 3.1, the
snapshot command behaves as before with the exception that it creates
an association between the original volume and the snap volume. A new
command, vxassist snapback leverages this association to expediently
return the snap mirror (M
Snap
) to the volume from which it was snapped
(in this example, V
Pri
).
Additionally, a new command relieves a volume of the administrative
overhead of tracking a snapshot (it would permanently destroy the
association created by the snapshot command). This capability is useful
in situations where it is known that a snapshot will never return to the
volume from which it was created. To achieve this result, the command
vxassist snapclear has been added to the family of snapshot
commands.