Automatic Minicopy on volume processing means that an existing
HBMM bitmap functions as a minicopy bitmap when connectivity to one
or more shadow set members is lost and is not restored during the
shadow member timeout period.
Before the introduction of this feature in OpenVMS Version 8.3,
it was a lengthy process to return expelled members to a shadow set
after connectivity was restored. The expelled members are returned
only by undergoing a full copy. The availability of a bitmap enables
the use of a minicopy operation, which takes less time than a full
When connectivity is lost, the shadow set is paused for volume
processing, that is, writes and reads are temporarily suspended until
connectivity is restored or the timeout period expires (established
by the value of SHADOW_MBR_TMO), whichever comes first. If connectivity
is not restored by the end of the timeout period, the member or members
are expelled from the shadow set, read and write I/O to the remaining
member or members resumes, and the bitmap keeps track of the writes.
The bitmap, whose name has changed from HBMMx to rrsex, functions
as a minicopy bitmap for the member or members that are expelled.
|NOTE: While one or two members are expelled and after all members
are restored to membership in the shadow set, the HBMM bitmap functionality
remains in effect. The HBMM bitmap functionality is useful in the
case of an expelled member only when the shadow set has three members
and one member is expelled.|
When connectivity is restored to one of the expelled shadow
set members, you can mount it back into the shadow set. If the expelled
member's metadata matches a bitmap that exists, it is used for a minicopy
operation to restore that member to the shadow set. If a second shadow
set member was removed at the same time, that member can also use
that bitmap. After the members are restored to the shadow set, the
name of the bitmap reverts to its HBMM bitmap name.
When one or more members are expelled from a shadow set, you
must minimize the time for the following reasons:
During a period of reduced membership of the shadow
set, data availability is at risk.
If a shadow set member is expelled, reads and writes
to the remaining members continue. The more writes that take place
before the expelled member or members are returned, the longer it
takes to restore the member or members to the shadow set. This is
especially significant in a disaster tolerant (DT) configuration.
To enable automatic bitmap creation on volume processing, you
must establish an HBMM policy for the shadow sets, and include the
new MULTIUSE keyword in the policy.