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

67Understanding Veritas Volume Manager

FastResync

Figure 1-33 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 61. 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 Figure 1-34, 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.

Associated with both the original volume and the snapshot volume are snap objects. The

snap object for the original volume points to the snapshot volume, and the snap object for

the snapshot volume points to the original volume. This allows VxVM to track the

Mirrored volume

Data plex

DCO volume

Data plex

Data change object

DCO

plex

Snapshot

plex

Disabled

DCO

plex