Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

64 Understanding Veritas Volume Manager
FastResync
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 463.
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 187
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 269 for a description of how to find
out the version number of a DCO that is associated with a volume.
Version 0 DCO volume layout
In VxVM releases 3.2 and 3.5, the DCO object only managed information about the
FastResync maps. These maps track writes to the original volume and to each of up to 32
snapshot volumes since the last
snapshot operation. Each plex of the DCO volume on
disk holds 33 maps, each of which is 4 blocks in size by default.
Persistent FastResync uses the maps in a version 0 DCO volume on disk to implement
change tracking. As for non-persistent FastResync, each bit in the map represents a region
(a contiguous number of blocks) in a volume’s address space. The size of each map can be
changed by specifying the dcolen attribute to the
vxassist command when the volume
is created. The default value of dcolen is 132 1024-byte 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 need to specify. For example, to use an 8-