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70 Understanding Veritas Volume Manager
FastResync
(by default) are used either for tracking writes to snapshots, or as copymaps.
The size of the DCO volume is determined by the size of the regions that are
tracked, 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. Associated with the volume are a DCO object
and a DCO volume with two plexes.