Veritas Volume Manager 4.1 Administrator's Guide (HP-UX 11i v3, February 2007)

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52 VERITAS Volume Manager Administrator’s Guide
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.