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: Mirrored volume with persistent FastResync enabled shows an example of a mirrored volume with two plexes on which Persistent FastResync is enabled.
Figure: Mirrored volume after completion of a snapstart operation shows how a snapshot plex is set up in the volume, and how a disabled DCO plex is associated with it.
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.
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: Mirrored volume and snapshot volume after completion of a snapshot operation shows how the creation of the snapshot volume also sets up a DCO object and a DCO volume for the snapshot volume.
The 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 space-optimized instant 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 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 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.