Veritas Volume Manager 5.0 Administrator's Guide (September 2006)

158 Creating and administering disk groups
Specifying a disk group to commands
groups at any time. Disks need not be added to disk groups until the disks are needed to
create VxVM objects.
When a disk is added to a disk group, it is given a name (for example, mydg02). This
name identifies a disk for operations such as volume creation or mirroring. The name also
relates directly to the underlying physical disk. If a physical disk is moved to a different
target address or to a different controller, the name mydg02 continues to refer to it. Disks
can be replaced by first associating a different physical disk with the name of the disk to
be replaced and then recovering any volume data that was stored on the original disk (from
mirrors or backup copies).
Having disk groups that contain many disks and VxVM objects causes the private region
to fill. In the case of large disk groups that are expected to contain more than several
hundred disks and VxVM objects, disks should be set up with larger private areas. A
major portion of a private region provides space for a disk group configuration database
that contains records for each VxVM object in that disk group. Because each
configuration record takes up approximately 256 bytes, the number of records that can be
created in a disk group can be estimated from the configuration database copy size. The
copy size in blocks can be obtained from the output of the command
vxdg list
diskgroup as the value of the permlen parameter on the line starting with the string
“config:”. This value is the smallest of the len values for all copies of the
configuration database in the disk group. The amount of remaining free space in the
configuration database is shown as the value of the free parameter. An example is
shown in “Displaying disk group information” on page 160. One way to overcome the
problem of running out of free space is to split the affected disk group into two separate
disk groups. See “Reorganizing the contents of disk groups” on page 185 for details.
For information on backing up and restoring disk group configurations, see “Backing up
and restoring disk group configuration data” on page 202.
Specifying a disk group to commands
Note: Most VxVM commands require superuser or equivalent privileges.
Many VxVM commands allow you to specify a disk group using the -g option. For
example, the following command creates a volume in the disk group, mktdg:
# vxassist -g mktdg make mktvol 5g
The block special device corresponding to this volume is:
/dev/vx/dsk/mktdg/mktvol