VERITAS Volume Manager 3.5 Administrator's Guide (September 2002)

Introduction

272 VERITAS Volume Manager Administrator’s Guide

FastResync of Volume Snapshots

Note You may need an additional license to use this feature.

VxVM allows you to take multiple snapshots of your data at the level of a volume. A

snapshot volumecontains a stable copy of avolume’s data at a given momentin time that

you can use for online backup or decision support. If FastResync is enabled on a volume,

VxVM usesa FastResync map to keep trackof which blocks areupdated in the volumeand

in the snapshot. If the data in one mirror is not updated for some reason, it becomes

out-of-date, or stale, with respect to the other mirrors in the volume. The presence of the

FastResync map means that only those updates that the mirror has missed need be

reapplied to resynchronize it with the volume. A full, and thereby much slower,

resynchronization of the mirror from the volume is unnecessary.

Two forms or FastResync may be conﬁgured on a volume: Persistent FastResync and

Non-Persistent FastResync. Persistent FastResync uses disk storage to ensure that

FastResync maps survive both system and cluster crashes. Non-Persistent FastResync

maps are held in memory. Non-Persistent FastResync maps for volumes in shared disk

groups can survive individual system crashes in a cluster but cannot survive cluster

crashes. Non-Persistent FastResync maps for volumes in private disk groups do not

survive if the system crashes that is accessing them.

When snapshot volumes are reattached to their original volumes, FastResync allows the

snapshot data tobe quicklyrefreshed and re-used. IfPersistent FastResync isenabled ona

volume in a private disk group, such incremental resynchronization can happen 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. When the disk groups are

rejoined, this allows the snapshot plexes to be quickly resynchronized. Non-Persistent

FastResync cannot be used for this purpose.

Note If you move or split anoriginal volumeinto aseparate disk group from its snapshot

volume, and then move orjoin the volumesinto the same disk group, you must use

the vxplex snapback command withthe -f option to resynchronizethe snapshot

plexes. You cannot use vxassist snapback for this purpose. This restriction does

not applyif you split a snapshot volumeinto a separate disk group from itsoriginal

volume, and subsequently return the snapshot volume to the original disk group.

For more information, see “Volume Snapshots” on page 42 and “FastResync” on page 43.