VERITAS Storage Foundation 4.1 Oracle Administrator's Guide

VERITAS Volume Manager Prerelease 8 September 2005, 8:54am
30 VERITAS Storage Foundation for Oracle Administrator’s Guide
Volume Snapshots
A volume snapshot is a point-in-time image of a volume. VERITAS Volume Manager provides
three volume snapshot features based on disk mirroring:
◆ Full-sized instant snapshots
◆ Space-optimized instant snapshots
◆ Emulation of third-mirror snapshots
For detailed information on performing database backups using volume snapshots, see “Using
Database FlashSnap for Backup and Off-Host Processing” on page 197
VERITAS FastResync (Optional)
VERITAS FastResync (previously called Fast Mirror Resynchronization or FMR) is included with
the Enterprise Edition. It is also included as part of the VERITAS FlashSnap option with the
Standard Edition.
VERITAS FastResync performs quick and efficient resynchronization of stale mirrors (mirrors that
are not synchronized). This increases the efficiency of the VxVM snapshot mechanism, and
improves the performance of operations such as backup and decision support. Typically, these
operations require that the volume is quiescent, and that they are not impeded by updates to the
volume by other activities on the system. To achieve these goals, the snapshot mechanism in
VxVM creates an exact copy of a primary volume at an instant in time. After a snapshot is taken, it
can be accessed independently of the volume from which it was taken.
VERITAS Storage Foundation for Oracle Enterprise Edition includes a feature called Database
FlashSnap, which takes advantage of the FastResync and disk group split and join features.
Database FlashSnap provides a quicker and easier way for database administrators to use volume
snapshots. For more information, see “VERITAS Database FlashSnap” on page 36.
Non-Persistent FastResync
Non-persistent FastResync allocates its change maps in memory. If non-persistent FastResync is
enabled, a separate FastResync map is kept for the original volume and for each snapshot volume.
Unlike a dirty region log (DRL), these maps do not reside on disk nor in persistent store. The
advantage is that updates to the FastResync map have little impact on I/O performance, as no disk
updates need to be performed. However, if a system is rebooted, the information in the map is lost,
so a full resynchronization is required when performing a snapback operation. This limitation
can be overcome for volumes in cluster-shareable disk groups, provided that at least one of the
nodes in the cluster remains running to preserve the FastResync map in its memory.