72Understanding Veritas Volume Manager

FastResync

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 tradeoff in functionality in using space-optimized snapshots as described in “Comparison of snapshot features” on page 65. 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 relationship between volumes and their snapshots even if they are moved into different disk groups.

The snap objects in the original volume and snapshot volume are automatically deleted in the following circumstances:

■For traditional snapshots, the vxassist snapback operation is run to return all of the plexes of the snapshot volume to the original volume.

■For traditional snapshots, the vxassist snapclear operation is run on a volume to break the association between the original volume and the snapshot volume. If the volumes are in different disk groups, the command must be run separately on each volume.

■For full-sized instant snapshots, the vxsnap reattach operation is run to return all of the plexes of the snapshot volume to the original volume.

■For full-sized instant snapshots, the vxsnap dis or vxsnap split operations are run on a volume to break the association between the original volume and the snapshot volume. If the volumes are in different disk groups, the command must be run separately on each volume.