Veritas Storage Foundation 5.1 SP1 Advanced Features Administrator"s Guide (5900-1503, April 2011)

When snapshot volumes are reattached to their original volumes, persistent
FastResync allows the snapshot data to be quickly refreshed and re-used. Persistent
FastResync uses disk storage to ensure that FastResync maps survive both system
and cluster crashes. If persistent FastResync is enabled on a volume in a private
disk group, incremental resynchronization can take place even if the host is
rebooted.
Persistent FastResync can track the association between volumes and their
snapshot volumes after they are moved into different disk groups. After the disk
groups are rejoined, persistent FastResync allows the snapshot plexes to be quickly
resynchronized.
See the Veritas Volume Manager Administrator’s Guide.
Instant volume snapshots
The traditional type of volume snapshot that was provided in VxVM is of the
third-mirror type. This name comes from its original implementation by adding
an additional plex to a mirrored volume. After the contents of the third-mirror
(or snapshot plex) had been synchronized from the original plexes of the volume,
it could be detached as a snapshot volume for use in backup or decision support
applications. Enhancements to the snapshot model allowed 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.
Release 4.0 of VxVM introduced instant snapshots, which offer advantages over
traditional third-mirror snapshots. The benefits of instant snapshots include
immediate availability for use, quick refreshment, and easier configuration and
administration. Full-sized instant snapshots are similar to third-mirror snapshots
in that they are the same length as the original volume.
Space-optimized instant snapshots require less space than full-sized snapshots
by recording changed regions in the original volume to a storage cache. As the
original volume is written to, VxVM preserves its data in the cache before the
write is committed.
See the Veritas Volume Manager Administrator’s Guide.
Data integrity in volume snapshots
A volume snapshot represents the data that exists in a volume at a given point in
time. As such, VxVM does not have any knowledge of data that is cached by the
overlying file system, or by applications such as databases that have files open in
the file system. If the fsgen volume usage type is set on a volume that contains a
Veritas File System (VxFS), intent logging of the file system metadata ensures the
internal consistency of the file system that is backed up. For other file system
135Understanding point-in-time copy methods
About point-in-time copy technology