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

Chapter 1, Understanding VERITAS Volume Manager
FastResync
51
DCO Volume Versioning
The internal layout of the DCO volume changed in VxVM 4.0 to support new features
such as full-sized and space-optimized instant snapshots. Because the DCO volume
layout is versioned, VxVM software continues to support the version 0 layout for legacy
volumes. However, you must configure a volume to have a version 20 DCO volume if you
want to take instant snapshots of the volume. Future releases of VERITAS Volume
Manager may introduce new versions of the DCO volume layout.
See “Determining the DCO Version Number” on page 238 for a description of how to find
out the version number of a DCO that is associated with a volume.
Version 0 DCO Volume Layout
In VxVM releases 3.2 and 3.5, 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.
Note 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 (see “Dirty Region Logging (DRL)” on
page 42) and special maps called copymaps that allow instant snapshot operations to
resume correctly following a system crash.