DDS, introduced in OpenVMS Version 7.3-2, allows you to construct
shadow sets of disk devices that are of dissimilar sizes.
Write bitmaps track application writes made to a shadow set
virtual unit so that a member can be returned to that virtual unit
without the overhead of a full copy. A write bitmap is created when
the user issues a DISMOUNT/POLICY=MINICOPY command
for a shadow set member or mounts a shadow set using the MOUNT/POLICY=MINICOPY command. When this bitmap is created,
its size depends on the current size of the volume.
When a shadow set is mounted, the logical size of the shadow
set virtual unit is set to the size of the smallest member unit. When
a member of the shadow set is removed, the logical size of the virtual
unit is recomputed based on the sizes of the remaining members of
the set. Consequently, the logical size of the virtual unit might
increase.
When a write bitmap is created for a shadow set, its size is
determined by the current size of the shadow set virtual unit. If
the virtual unit’s size subsequently increases, the bitmap
does not cover the entire virtual unit. If the bitmap is then used
to bring back a shadow set member with a minicopy operation, the portion
of the virtual unit that is not covered by the bitmap is copied with
a full copy operation.
The following example illustrates this problem:
Shadow set DSA1: includes the following three members:
$1$DGA20: (18 GB)
$1$DGA21: (36 GB)
$1$DGA22: (36 GB)
$1$DGA22: is removed from the shadow set with a minicopy
bitmap using the following command:
$ DISMOUNT/POLICY=MINICOPY
$1$DGA22:
The write bitmap is sized for 18 GB, the current size of the
shadow set virtual unit.
$1$DGA20: is removed from the shadow set. To allow
the file system to utilize the entire 36 GB of the remaining member,
use the following command:
$ SET VOLUME/SIZE
DSA1
$1$DGA20 can no longer be used in this shadow set because it
is smaller than the new volume size.
$1$DGA22: is returned to the shadow set using the
following command:
$ MOUNT/SYSTEM DSA1:/SHADOW=$1$DGA22:
label
The logical size of DSA1: remains at 36 GB; however, the bitmap
covers only the first 18 GB.
The first 18 GB of $1$DGA22: are copied using the
minicopy bitmap; the remaining 18 GB are copied using a full copy
operation.
If the removal of a smaller shadow set member is planned,
removing it before removing a larger member with a minicopy bitmap
causes a larger bitmap to be created and avoids the performance impact
of a short bitmap. (In the preceding example, you would remove $1$DGA20:
before removing $1$DGA22:.)
