FastResync optimizes mirror resynchronization by keeping track of updates to stored data that have been missed by a mirror. If FastResync is enabled on a volume, VxVM uses a FastResync map to keep track of which blocks are updated in the volume and in the snapshot. If the data in one mirror is not updated for some reason, it becomes out-of-date, or stale, with respect to the other mirrors in the volume. The presence of the FastResync map means that only those updates that the mirror has missed need be reapplied to resynchronize it with the volume. A full (and therefore much slower) resynchronization of the mirror from the volume is unnecessary. The FastResync feature increases the efficiency of the VxVM snapshot mechanism to better support operations such as backup and decision support.

The persistent form of FastResync ensures that FastResync maps survive both system crashes and cluster restarts. When snapshot volumes are reattached to their original volumes, FastResync allows the snapshot data to be quickly refreshed and re-used. If Persistent FastResync is enabled on a volume in a private disk group, such incremental resynchronization can happen even if the host is rebooted.

Persistent FastResync can track the association between volumes and their snapshot volumes after they are moved into different disk groups. When the disk groups are rejoined, this allows the snapshot plexes to be quickly resynchronized. Non-Persistent FastResync cannot be used for this purpose.

FastResync allows you to refresh and re-use snapshots rather than discard them. You can quickly resynchronize a snapshot volume with its original volume. This reduces the system overhead required to perform cyclical operations, such as backups, that rely on the snapshot functionality of VxVM. You can also resynchronize the original volume from the snapshot volume. In this case, the database must be shut down and all of the file systems on the original volumes must be unmounted.

Up to 31 snapshot mirrors can be taken and tracked via FastResync.

For more information about FastResync, see the Veritas Volume Manager Administrator's Guide.