If FastResync is enabled on a volume before a snapshot is taken, the snapshot feature of VxVM uses FastResync change tracking to record updates to the original volume after a snapshot plex is created. When the snapback option is used to reattach the snapshot plex, the changes that FastResync recorded are used to resynchronize the volume during the snapback. This behavior considerably reduces the time needed to resynchronize the volume.

Non-persistent FastResync uses a map in memory to implement change tracking. The map does not exist on disk or in persistent store. The advantage of non-persistent FastResync is that updates to the FastResync map have little impact on I/O performance, because no disk updates are performed. However, FastResync must remain enabled until the snapshot is reattached, and the system cannot be rebooted. If FastResync is disabled or the system is rebooted, the information in the map is lost and a full resynchronization is required on snapback.

This limitation can be overcome for volumes in cluster-shareable disk groups, provided that at least one of the nodes in the cluster remained running to preserve the FastResync map in its memory. However, a node crash in a High Availability (HA) environment requires the full resynchronization of a mirror when it is reattached to its parent volume.

Each bit in the FastResync map represents a contiguous number of blocks in a volume's address space. The default size of the map is 4 blocks. The kernel tunable vol_fmr_logsz can be used to limit the maximum size in blocks of the map

For information about tuning VxVM, see the Storage Foundation and High Availability Solutions Tuning Guide.