Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

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 “Tunable parameters for VxVM” on page 496.
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 250.
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.
DCO volume versioning
The internal layout of the DCO volume changed in VxVM 4.0 to support new
features such as full-sized and space-optimized instant snapshots, and a unified
DRL/DCO. Because the DCO volume layout is versioned, VxVM software continues
to support the version 0 layout for legacy volumes. However, you must configure
a volume to have a version 20 DCO volume if you want to take instant snapshots
of the volume. Future releases of Veritas Volume Manager may introduce new
versions of the DCO volume layout.
See “Determining the DCO version number” on page 362.
65Understanding Veritas Volume Manager
FastResync