Starting with OpenVMS
Alpha Version 7.3–2, different size devices can be used to
form a shadow set. 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 Integrity
serversVersion 8.2 or later or 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 is 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.
All members of the shadow
set must have a MAXBLOCK size equal to or greater than the logical
volume size stored in the storage control block SCB$L_VOLSIZE of the
shadow set. All mounted members have this value. When the smaller
volume is no longer needed, or if you have to increase the file system
size of the shadow set, you can use the dynamic volume expansion (DVE)
feature introduced in OpenVMS Alpha Version 7.3–2. Together,
the features of DDS and DVE enable you to continually grow a logical
volume without ever having to take it offline again. For more information
about DVE, see “Dynamic Volume Expansion (Integrity servers and Alpha)”.
|NOTE: In volume expansion, you must disable and re-enable HBMM so
that write bitmaps are recreated that encompasses the new volume size.
Failing to do this might result in longer than expected merge times
as the expansion area is subjected to a complete merge.|
You can determine the block size for each disk
with the SHOW DEVICE /FULL command. The block size is displayed as Total blocksn.