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

Associated with the volume are a DCO object and a DCO volume with two plexes.
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.
Figure 1-33 shows how a snapshot plex is set up in the volume, and how a disabled
DCO plex is associated with it.
Figure 1-33
Mirrored volume after completion of a snapstart operation
Data plex Data plex Data plex
Disabled
DCO plex
Mirrored volume
DCO plex DCO plex
DCO volume
Data change object
Multiple snapshot plexes and associated DCO plexes may be created in the volume
by re-running the vxassist snapstart command for traditional snapshots, or
the vxsnap make command for space-optimized snapshots. You can create up to
a total of 32 plexes (data and log) in a volume.
Space-optimized instant snapshots do not require additional full-sized plexes to
be created. Instead, they use a storage cache that typically requires only 10% of
the storage that is required by full-sized snapshots. There is a trade-off in
functionality in using space-optimized snapshots. The storage cache is formed
within a cache volume, and this volume is associated with a cache object. For
convenience of operation, this cache can be shared by all the space-optimized
instant snapshots within a disk group.
See “Comparison of snapshot features” on page 62.
A traditional snapshot volume is created from a snapshot plex by running the
vxassist snapshot operation on the volume. For instant snapshots, however,
the vxsnap make command makes an instant snapshot volume immediately
available for use. There is no need to run an additional command.
Figure 1-34 shows how the creation of the snapshot volume also sets up a DCO
object and a DCO volume for the snapshot volume.
Understanding Veritas Volume Manager
FastResync
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