Instant snap DCO volume layout

Previously known as Version 20 DCO volume layout, this version of the DCO layout supports instant snapshots of volumes.

This type of DCO manages the FastResync maps, and also manages DRL recovery maps and special maps called copymaps that allow instant snapshot operations to resume correctly following a system crash.

Version 0 DCO volume layout

This version of the DCO volume layout only supports legacy snapshots (vxassist snapshots). The DCO object manages information about the FastResync maps. These maps track writes to the original volume and to each of up to 32 snapshot volumes since the last snapshot operation. Each plex of the DCO volume on disk holds 33 maps, each of which is 4 blocks in size by default.

VxVM software continues to support the version 0 (zero) layout for legacy volumes.