Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

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54 VERITAS Volume Manager Administrator’s Guide
Mirrored Volume After Completion of a snapstart Operation
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.
Note 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 as described in “Comparison of Snapshot
Features” on page 47. 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 instant space-optimized snapshots within a disk group.
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.
As illustrated in “Mirrored Volume and Snapshot Volume After Completion of a snapshot
Operation” on page 56, creation of the snapshot volume also sets up a DCO object and a
DCO volume for the snapshot volume. This DCO volume contains the single DCO plex
that was associated with the snapshot plex. If two snapshot plexes were taken to form the
snapshot volume, the DCO volume would contain two plexes. For instant
space-optimized snapshots, the DCO object and DCO volume are associated with a
snapshot volume that is created on a cache object and not on a VM disk.
Mirrored Volume
Data Plex
DCO Volume
Data Plex
Data Change Object
DCO
DCO
Plex
Plex
Snapshot
Plex
Disabled
DCO
Plex