HP Veritas Volume Manager 5.0 -UX 11i v3 manual Original

64Understanding Veritas Volume Manager

Volume snapshots

Figure 1-31

Volume snapshot as a point-in-time image of a volume

T1

T2

T3

T4

Time

Original volume

Original		Snapshot
volume		volume
Original		Snapshot
volume		volume
Original		Snapshot
volume		volume
	Resynchronize

snapshot volume

from original volume

Snapshot volume is created at time T2

Snapshot volume retains image taken at time T2

Snapshot volume is updated at time T4

The traditional type of volume snapshot in VxVM is of the third-mirrorbreak-offtype. This name comes from its implementation where a snapshot plex (or third mirror) is added to a mirrored volume. The contents of the snapshot plex are then synchronized from the original plexes of the volume. When this synchronization is complete, the snapshot plex can be detached as a snapshot volume for use in backup or decision support applications. At a later time, the snapshot plex can be reattached to the original volume, requiring a full resynchronization of the snapshot plex’s contents. For more information about this type of snapshot, see “Traditional third-mirrorbreak-off snapshots” on page 305.

The FastResync feature was introduced to track writes to the original volume. This tracking means that only a partial, and therefore much faster, resynchronization is required on reattaching the snapshot plex. In later releases, the snapshot model was enhanced to allow snapshot volumes to contain more than a single plex, reattachment of a subset of a snapshot volume’s plexes, and persistence of FastResync across system reboots or cluster restarts.

For more information about FastResync, see “FastResync” on page 66.

Release 4.0 of VxVM introduced full-sized instant snapshots and space- optimized instant snapshots, which offer advantages over traditional third-