The instant snap data change object (DCO) supports full-sized and space-optimized instant snapshots. Traditional third-mirror volume snapshots that are administered using the vxassist command are not supported with this DCO layout.

Introduced in Veritas Volume Manager (VxVM) 4.0, the instant snap DCO volume layout is also known as a version 20 DCO volume layout. This type of DCO 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.

In Storage Foundation Cluster File System High Availability 6.0, the volume layout of an instant snap DCO has been changed to improve the I/O performance and scalability of instant snapshots. The change in layout does not alter how you administer instant snapshots. The only visible affect is in improved I/O performance and in some cases, increased size of a DCO volume.

The layout of an instant snap DCO volume uses dynamic creation of maps on the preallocated storage. The size of the DRL (Dirty region logging) map does not depend on volume size. You can configure the size of the DRL by using the option drlmapsz while creating the DCO volume. By default, the size of the DRL is 1MB.

For CVM configurations, each node has a dedicated DRL map that gets allocated during the first write on that node. By default, the size of the DCO volume accommodates 32 DRL maps, an accumulator, and 16 per-volume maps (including a DRL recovery map, a detach map to track detached plexes, and the remaining 14 maps for tracking snapshots).

The size of the DCO plex can be estimated using the following formula:

DCO_volume_size = (32*drlmapsize + acmsize + 16*per-volume_map_size)

where:

acmsize = (volume_size / (region_size*4))

per-volume_map_size = (volume_size/region_size*8)

drlmapsize = 1M, by default 

For a 100GB volume, the size of the DCO volume with the default regionsize of 64KB is approximately 36MB.

Create the DCOs for instant snapshots by using the vxsnap prepare command or by specifying the options logtype=dco dcoversion=20 while creating a volume with the vxassist make command.