Version 20 DCO volume layout

In VxVM 4.0 and later releases, the DCO object is used not only to manage the FastResync maps, but also to manage DRL recovery maps and special maps called copymaps that allow instant snapshot operations to resume correctly following a system crash.

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.


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.

More Information

Dirty region logging