Veritas Volume Manager 5.1 SP1 Administrator"s Guide (5900-1506, April 2011)

Version 0 DCO volume layout

In earlier releases of VxVM, the DCO object only managed 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.

Persistent FastResync uses the maps in a version 0 DCO volume on disk to

implement change tracking. As for non-persistent FastResync, each bit in the

map represents a region (a contiguous number of blocks) in a volume’s address

space. The size of each map can be changed by specifying the dcolen attribute to

the vxassist command when the volume is created. The default value of dcolen

is 132 1024-byte blocks (the plex contains 33 maps, each of length 4 blocks). To

use a larger map size, multiply the desired map size by 33 to calculate the value

of dcolen that you need to specify. For example, to use an 8-block map, you would

specify dcolen=264. The maximum possible map size is 64 blocks, which

corresponds to a dcolen value of 2112 blocks.

The size of a DCO plex is rounded up to the nearest integer multiple of the disk

group alignment value. The alignment value is 8KB for disk groups that support

the Cross-platform Data Sharing (CDS) feature. Otherwise, the alignment value

is 1 block.

Only traditional (third-mirror) volume snapshots that are administered using the

vxassist command are supported for the version 0 DCO volume layout. Full-sized

and space-optimized instant snapshots are not supported.

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.

See “Dirty region logging” on page 57.

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,

Understanding Veritas Volume Manager

FastResync