On OpenVMS Version 8.3 and later, you can implement HBMM write
bitmaps to perform minicopies, under certain conditions. By specifying
the MULTIUSE property, you can prevent full copies from occurring
when a member is removed from a shadow set as a result of loss of
connectivity (For example, a MEDOFFL occurs when a link to the remote
site is lost). If MULITUSE cannot initiate a minicopy, it reverts
back to a full copy to ensure that the data is safe.
To use minicopies using the MULTIUSE property:
Ensure that all VMS cluster members are running OpenVMS
Version 8.3 or higher with the latest volume shadowing kits.
Enable HBMM; the Multiuse property uses HBMM write
Define an HBMM policy where you specify the Multiuse
$ SET SHADOW DSAnnn/POLICY=HBMM=((Master=(Node1, Node2), Count=2, Multiuse=1), -
(Master=(Node3, Node4), Count=2, Multiuse=2) )
In the syntax:
Node1 and Node2 are at Site A
Node3 and Node4 are at Site B
Count refers to the number of master
bitmaps that are created at the site. This number must be less than
or equal to the number of nodes listed. the total number of master
bitmaps is limited to six. This can be increased up to a maximum of
12 using the DISMOUNT=n keyword. For more information on the DISMOUNT
keyword, see the “SET SHADOW Command Qualifiers” and also “Examples of Multiuse and Dismount”.
Multiuse refers to the number of
master bitmaps that can be converted to multiuse bitmaps, when a member
is automatically removed from the shadow set. This number must be less than or equal to the COUNT for that site.
When a shadow set member is removed, of the two master HBMM
bitmaps created for a shadow set that uses this policy, only one of
the two HBMM master bitmaps at Site A is converted to multiuse bitmaps.
At Site B, both the master bitmaps are eligible for use as multiuse
Until a member is removed from the set, output from the $SHOW
DEVICE/BITMAP command shows the bitmap as a minimerge bitmap. It is
not marked as multiuse until a member is removed and the bitmap is
A multiuse bitmap can be used to perform an HBMM recovery (if
a node fails and causes a merge) or a multiuse bitmap can be used
to bring the former member back into the shadow set using a minicopy.
A multiuse bitmap cannot use minicopy to bring a new member into
the set. In addition, if the disk is removed due to a fatal drive
error, then the bitmaps are not converted to multiuse and it is unlikely
that the broken disk is returned.
Multiuse Property and DISMOUNT Keyword
The DISMOUNT keyword specifies the number of HBMM bitmaps to
be converted to multiuse bitmaps during the manual removal of members.
If MULTIUSE is omitted, then automatic minicopy on volume processing
is not enabled. As a result, no HBMM bitmap is converted to multiuse
bitmap. If DISMOUNT is omitted, only a maximum of 6 HBMM bitmaps can
be used as multiuse bitmaps.
For examples on the DISMOUNT keyword, see the “Examples of Multiuse and Dismount”.