VERITAS Volume Manager 4.1 Administrator's Guide

Understanding VERITAS Volume Manager
FastResync
Chapter 154
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 172 for details.
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 shapshot 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.
How Persistent FastResync Works with Snapshots
Persistent FastResync uses a map in a DCO volume on disk to
implement change tracking. As for Non-Persistent FastResync, each bit
in the map represents a contiguous number of blocks in a volume’s
address space. The default size of the map is 1 block. This can be
increased by specifying the dcolen attribute to the vxassist command
when the volume is created. The default value of dcolen is 132 1024-byte
blocks (the plex contains 33 maps, each of length 4 blocks). To use a
larger map size, multiply the desired map size by 33 to calculate the
value of dcolen that you should specify. For example, to use an 8-block
map, you would specify dcolen=264. The maximum possible map size is
64 blocks, which corresponds to a dcolen value of 2112 blocks.