Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

Non-persistent FastResync
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.
How non-persistent FastResync works with snapshots
The snapshot feature of VxVM takes advantage of FastResync change tracking
to record updates to the original volume after a snapshot plex is created. After a
snapshot is taken, the snapback option is used to reattach the snapshot plex.
Provided that FastResync is enabled on a volume before the snapshot is taken,
and that it is not disabled at any time before the snapshot is reattached, the
changes that FastResync records are used to resynchronize the volume during
the snapback. This considerably reduces the time needed to resynchronize the
volume.
Non-Persistent FastResync uses a map in memory to implement change tracking.
Each bit in the 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.
See “DMP tunable parameters ” on page 540.
Persistent FastResync
Unlike non-persistent FastResync, persistent FastResync keeps the FastResync
maps on disk so that they can survive system reboots, system crashes and cluster
crashes. Persistent FastResync can also 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. This ability is not supported by non-persistent FastResync.
See “Reorganizing the contents of disk groups” on page 227.
If persistent FastResync is enabled on a volume or on a snapshot volume, a data
change object (DCO) and a DCO volume are associated with the volume.
67Understanding Veritas Volume Manager
FastResync