VERITAS Volume Manager 4.1 Administrator's Guide

Conﬁguring Off-Host Processing

Introduction

Chapter 11384

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 volume contains a stable copy of a volume’s data at a

given moment in time that you can use for online backup or decision

support. If FastResync is enabled on a volume, VxVM uses a FastResync

map to keep track of which blocks are updated in the volume and 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becomes out-of-date, or stale, with respect to the other mirrors in the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 to be quickly refreshed and re-used.

If Persistent FastResync is enabled on a 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 an original volume into a separate disk group from

its snapshot volume, and then move or join the volumes into the same

disk group, you must use the vxplex snapback command with the -f