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67Understanding Veritas Volume Manager

FastResync

snapshot is taken, it can be accessed independently of the volume from which it

was taken. In a clustered VxVM environment with shared access to storage, it is

possible to eliminate the resource contention and performance overhead of

using a snapshot simply by accessing it from a different node.

For details of how to enable FastResync on a per-volume basis, see “Enabling

FastResync on a volume” on page 292.

FastResync enhancements

FastResync provides two fundamental enhancements to VxVM:

■ FastResync optimizes mirror resynchronization by keeping track of updates

to stored data that have been missed by a mirror. (A mirror may be

unavailable because it has been detached from its volume, either

automatically by VxVM as the result of an error, or directly by an

administrator using a utility such as

vxplex or vxassist. A returning

mirror is a mirror that was previously detached and is in the process of

being re-attached to its original volume as the result of the

vxrecover or

vxplex att operation.) When a mirror returns to service, only the updates

that it has missed need to be re-applied to resynchronize it. This requires

much less effort than the traditional method of copying all the stored data

to the returning mirror.

Once FastResync has been enabled on a volume, it does not alter how you

administer mirrors. The only visible effect is that repair operations

conclude more quickly.

■ FastResync allows you to refresh and re-use snapshots rather than discard

them. You can quickly re-associate (snapback) snapshot plexes with their

original volumes. This reduces the system overhead required to perform

cyclical operations such as backups that rely on the snapshot functionality

of VxVM.

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),

they do not reside on disk nor in persistent store. This has the advantage that

updates to the FastResync map have little impact on I/O performance, as no disk

updates needed to be performed. However, if a system is rebooted, the

information in the map is lost, so a full resynchronization is required on

snapback. This limitation can be overcome for volumes in cluster-shareable disk

groups, provided that at least one of the nodes in the cluster remained running

to preserve the FastResync map in its memory. However, a node crash in a High

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HP 11i Manual do Utilizador Página 66