HP OpenVMS Systems Documentation

Content starts here Automatic Minicopy on Volume Processing
HP Volume Shadowing for OpenVMS: OpenVMS Version 8.4 > Chapter 8 Host-Based Minimerge (HBMM)

Automatic Minicopy on Volume Processing

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 copy operation.

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.