Dissimilar
Device Shadowing and Dynamic Volume Expansion
OpenVMS
Alpha Version 7.3-2 supports the shadowing of dissimilar devices
that differ in size. This functionality is called dissimilar device
shadowing (DDS). To use DDS, all systems that have mounted a shadow
set whose members differ in size must be running OpenVMS Alpha Version
7.3-2.
Prior to OpenVMS Alpha Version 7.3-2, Volume Shadowing for
OpenVMS required that all members of a shadow set be the same size,
that is, that each member have the exact same number of blocks.
The rapid advance of disk technology has made this requirement impractical.
The flexibility of using different size devices outweighs the space
that will be unused on the larger device.
Operationally, shadowing dissimilar devices means that you
can add a larger disk device to an existing shadow set. The shadow
set retains the file system size of the original shadow set. After
adding a larger disk, if you remove a smaller disk, the geometry
(cylinders, tracks, and sections) of the shadow set changes to the smallest
remaining disk, but the logical volume size (that is, the file system
size) is not changed.
A complementary new feature, dynamic volume expansion (DVE),
enables you to increase the logical volume size (amount of disk
space allocated to the file system) without dismounting the device.
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 (that is, 1 TB, if you want). This can be performed
at disk initialization time or by using the SET VOLUME command.
This makes it possible to expand the volume size at a later time
while the device is mounted. Note that to use the SET VOLUME command
to increase the expansion limit, the disk must be mounted privately.
However, once the expansion limit is set, the volume can be expanded
while the disk is mounted as shareable (MOUNT/SHARE).
Together, the features of dissimilar device shadowing and
dynamic volume expansion enable you to continually grow a logical
volume without ever having to take it off line again.
For more information about shadowing dissimilar devices, refer
to HP Volume Shadowing for OpenVMS. For information about dynamic volume expansion, refer
to the HP OpenVMS System Manager's Manual.
HP OpenVMS Alpha Version 7.3-2 New Features and... 
OpenVMS Alpha Version 7.3-2 New Features
