Veritas Volume Manager 5.0.1 Administrator's Guide, HP-UX 11i v3, First Edition, November 2009

See “Dirty region logging” on page 60.
Each bit in a map represents a region (a contiguous number of blocks) in a volume’s
address space. A region represents the smallest portion of a volume for which
changes are recorded in a map. A write to a single byte of storage anywhere within
a region is treated in the same way as a write to the entire region.
The layout of a version 20 DCO volume includes an accumulator that stores the
DRL map and a per-region state map for the volume, plus 32 per-volume maps
(by default) including a DRL recovery map, and a map for tracking detaches that
are initiated by the kernel due to I/O error. The remaining 30 per-volume maps
(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.
69Understanding Veritas Volume Manager
FastResync