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.

	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.

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.

	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.

Using the /SIZE Qualifier With the INITIALIZE Command

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

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.