The basis of dynamic volume expansion is the one-time allocation
of extra bitmap space to the maximum size that will ever be used
on this volume. The current limit is 1 TB. The one-time
allocation of extra bitmap space can be performed either at disk
initialization time with the INITIALIZE/LIMIT command or on a mounted volume
with the SET VOLUME/LIMIT command. By allocating extra bitmap
space,you can later expand the logical volume size while the device
is mounted by using SET VOLUME volume-name/SIZE=xxx command. (The
logical volume size is the amount of disk space allocated to the
file system.) For example, you might prepare a disk
for 1 TB of storage (by allocating 1 TB of bitmap space) but use
only 18 GB today. Next year, you might increase it to 36 GB, and
so on, until you reach the maximum of 1 TB. By allocating the maximum size
for storage on the disk, you can later increase the size of the
volume without stopping the application or dismounting a disk.To
use the SET VOLUME/LIMIT command to allocate extra bitmap space,
the disk must be mounted privately. However, once allocated, the
volume can be expanded while the disk is mounted as shareable (MOUNT/SHARE).
You can allocate additional bitmap space whether or not the
physical volume has room for expansion. The commands for allocating
extra bitmap size and for expanding the volume size are available
in OpenVMS Alpha Version 7.3-2. Volumes that use DVE can
be used by any AlphaServer or VAX system running OpenVMS Version
7.2 or later. The following command allocates extra bitmap size
on a new volume:
The following command allocates extra bitmap size on a mounted
volume:
$ SET VOLUME/LIMIT $1$DGAnnn
The default /LIMIT size for both commands is 1 TB, which is
also the maximum size currently supported on OpenVMS. In special
circumstances, you may want to specify less.
When additional physical storage is made available (either
by adding a larger device to the shadow set and removing the smaller
member, or by increasing the size on the storage subsystem), you
can then enter the following command to increase the volume size:
$ SET VOLUME $1$DGAnnn/SIZE=xxxx
In this command syntax, xxxx represents the number of blocks.
If the volume of a shadow set is expanded to be larger
than the physical size of a member, the smaller member can no longer
be added back to the shadow set.
Using the
/SIZE Qualifier With the INITIALIZE Command You can use the /SIZE qualifier to create a file system that
is smaller than the current physical size of the volume. If you
have a 36-GB disk and you anticipate adding an 18-GB disk in the
future, then you could initialize the disk with the following command:
$ INIT/SIZE=36000000 $1$DGAnnn
When to
Increase the Expansion Limit on Each Volume If you are adding a new volume to your system, increase the
expansion limit on the volume when you initialize the disk with
INITIALIZE/LIMIT. To increase the expansion limit on volumes already
in use, plan to increase the expansion limit during the next convenient
maintainance period using the command SET VOLUME/LIMIT.
When you use the /LIMIT qualifier with the INITIALIZE or SET
VOLUME command, you increase the BITMAP.SYS file by a few hundred
blocks, which gives you much greater flexibility in the future.
(If INITIALIZE/LIMIT is used, the default cluster size (for /CLUSTER_SIZE)
is 8. This value controls how much space the bitmap occupies.) You
can later expand the volume (using the SET VOLUME volume-name/SIZE=xxxx command)
while the device is still mounted if your storage requirements grow unexpectedly.