The basis of dynamic volume expansion is the one-time
allocation of extra bitmap space to the maximum size ever 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=x 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 regardless
of whether the physical volume has room for expansion. The commands
for allocating extra bitmap size and for expanding the volume size
are available in OpenVMS Integrity servers, starting with Version
8.2 and in OpenVMS Alpha Version, starting with Version 7.3–2.
Volumes that use DVE can be used by any Integrity
server system running OpenVMS Version 8.2 or later or by any Alpha system
running OpenVMS Version 7.2 or later.
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 | NOTE: In volume expansion, you must disable and re-enable HBMM so
that write bitmaps are recreated, which encompasses the new volume
size. Failing to do this might result in longer than expected merge
times because the expansion area is subject to a complete merge. |
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The following command allocates extra bitmap size
on a new volume:
$ INITIALIZE/LIMIT $1$DGAnnn: ! Allocates 1 TB bitmap |
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 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.
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 $1$DGAn/SIZE=x |
In this command syntax, x represents the number of blocks.
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| NOTE: 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. |
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Using the /SIZE Qualifier With the INITIALIZE Command |
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You can use the INITIALIZE/SIZE command 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 can initialize the disk with the following
command:
$ INIT/SIZE=36000000 $1$DGAnlabel |
In this example, 18GB disk = 36,000,000 blocks
of 512 bytes each approximately.
When to Increase the Expansion Limit on Each Volume |
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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 maintenance period using the command SET VOLUME/LIMIT.
If INITIALIZE/LIMIT is used, the default cluster
size of the /CLUSTER_SIZE qualifier is 8. This value controls how
much space the bitmap occupies. You can later expand the volume (using
the SET VOLUME volume-name/SIZE=x command)
while the device is still mounted if your storage requirements grow
unexpectedly.