Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

66 Understanding Veritas Volume Manager

FastResync

Note: Full-sized and space-optimized instant snapshots, which are administered using the

vxsnap command, are supported for a version 20 DCO volume layout. The use of the

vxassist command to administer traditional (third-mirror break-off) snapshots is not

supported for a version 20 DCO volume layout.

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.

Figure 1-32 shows an example of a mirrored volume with two plexes on which Persistent

FastResync is enabled. Associated with the volume are a DCO object and a DCO volume

with two plexes.

Figure 1-32 Mirrored volume with persistent FastResync enabled

To create a traditional third-mirror snapshot or an instant (copy-on-write) snapshot, the

vxassist snapstart or vxsnap make operation respectively is performed on the

volume. This sets up a snapshot plex in the volume and associates a disabled DCO plex

with it, as shown in Figure 1-33.

Mirrored volume

Data plex

DCO volume

Data plex

Data change object

DCO

plex