Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

FastResync
50 VERITAS Volume Manager Administrator’s Guide
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 Availability
(HA) environment requires the full resynchronization of a mirror when it is reattached to
its parent volume.
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 snapshot 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 FastResync uses a map in memory to implement 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 “Tunable Parameters” on
page 411.
Persistent FastResync
Unlike Non-Persistent FastResync, Persistent FastResync keeps the FastResync maps on
disk so that they can survive system reboots, system crashes and cluster crashes.
Persistent FastResync can also 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. This ability is not
supported by Non-Persistent FastResync. See “Reorganizing the Contents of Disk
Groups” on page 155 for details.
If Persistent FastResync is enabled on a volume or on a snapshot volume, a data change
object (DCO) and a DCO volume are associated with the volume.