Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

and by the number of per-volume maps. Both the region size and the number of
per-volume maps in a DCO volume may be configured when a volume is prepared
for use with snapshots. The region size must be a power of 2 and be greater than
or equal to 16KB.
As the accumulator is approximately 3 times the size of a per-volume map, the
size of each plex in the DCO volume can be estimated from this formula:
DCO_plex_size = ( 3 + number_of_per-volume_maps ) * map_size
where the size of each map in bytes is:
map_size = 512 + ( volume_size / ( region_size * 8 ))
rounded up to the nearest multiple of 8KB. Note that each map includes a 512-byte
header.
For the default number of 32 per-volume maps and region size of 64KB, a 10GB
volume requires a map size of 24KB, and so each plex in the DCO volume requires
840KB of storage.
Note: Full-sized and space-optimized instant snapshots, which are administered
using the vxsnap command, are supported for a version 20 DCO volume layout.
The use of the vxassist command to administer traditional (third-mirror
break-off) snapshots is not supported for a version 20 DCO volume layout.
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.
Figure 1-32 shows an example of a mirrored volume with two plexes on which
Persistent FastResync is enabled.
Figure 1-32
Mirrored volume with persistent FastResync enabled
Data plex Data plex
Mirrored volume
DCO plex DCO plex
DCO volume
Data change object
67Understanding Veritas Volume Manager
FastResync