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

Chapter 1, Understanding VERITAS Volume Manager

FastResync

How Non-Persistent FastResync Works with Snapshots

The snapshot feature of VxVM takes advantage of FastResync change tracking to record

updates to the original volume after a shapshot plex is created. After a snapshot is taken,

the snapback option is used to reattach the snapshot plex. Provided that FastResync is

enabled on a volume before the snapshot is taken, and that it is not disabled at any time

before the snapshot is reattached, the changes that FastResync records are used to

resynchronize the volume during the snapback. This considerably reduces the time

needed to resynchronize the volume.

Non-Persistent FastResyncuses a map in memory toimplement change tracking. Each bit

in the map represents a contiguous number of blocks in a volume’s address space. The

default size of the map is 4 blocks. The kernel tunable vol_fmr_logsz can be used to

limit the maximum size in blocks of the map as described on page 294.

How Persistent FastResync Works with Snapshots

Persistent FastResync uses a map in a DCO volume on disk to implement change

tracking. As for Non-Persistent FastResync, each bit in the map represents a contiguous

number of blocks in a volume’s address space. The default size of the map is 1 block. This

can be increased by specifyingthe dcolen attribute tothe vxassist command when the

volume is created. The default value of dcolen is 132 blocks (the plex contains 33 maps,

each of length 4 blocks). To use a larger map size, multiply the desired map size by 33 to

calculate the value of dcolen that you should specify. For example, to use an 8-block

map, you would specify dcolen=264. The maximum possible map size is 64 blocks,

which corresponds to a dcolen value of 2112 blocks.

“Mirrored Volume with Persistent FastResync Enabled” shows an example of a mirrored

volume with two plexes on which Persistent FastResync is enabled. Associated with the

volume are a DCO object and a DCO volume with two plexes.

Mirrored Volume with Persistent FastResync Enabled

Mirrored Volume

Data Plex

DCO Volume

Data Plex

Data Change Object

DCO

Plex