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68 Understanding Veritas Volume Manager
FastResync
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 475.
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 195 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.
DCO volume versioning
The internal layout of the DCO volume changed in VxVM 4.0 to support new
features such as full-sized and space-optimized instant snapshots. Because the
DCO volume layout is versioned, VxVM software continues to support the
version 0 layout for legacy volumes. However, you must configure a volume to
have a version 20 DCO volume if you want to take instant snapshots of the
volume. Future releases of Veritas Volume Manager may introduce new versions
of the DCO volume layout.
See “Determining the DCO version number” on page 277 for a description of
how to find out the version number of a DCO that is associated with a volume.