Non-persistent FastResync allocates its change maps in memory. They do not reside on disk nor in persistent store. This has the advantage that updates to the FastResync map have little impact on I/O performance, as no disk updates needed to be performed. However, if a system is rebooted, the information in the map is lost, so 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.