HP OpenVMS Systems Documentation

Content starts here Use of /DEMAND_MERGE When HBMM Is Enabled
HP Volume Shadowing for OpenVMS: OpenVMS Version 8.4 > Chapter 8 Host-Based Minimerge (HBMM)

Use of /DEMAND_MERGE When HBMM Is Enabled

If a shadow set is HBMM-enabled and is actively using HBMM, then the SET SHADOW/DEMAND_MERGE DSAn: command causes a minimerge operation to occur. To force a full merge instead of a minimerge operation, you must disable HBMM on the shadow set before issuing the SET SHADOW/DEMAND_MERGE DSAn: command. For information about disabling HBMM, see “How to Disable HBMM on a Shadow Set”

The /DEMAND_MERGE qualifier of the SET SHADOW command is used primarily to force a merge operation on shadow sets that are created with the INITIALIZE/SHADOW command without specifying the /ERASE qualifier. The /DEMAND_MERGE qualifier ensures that all blocks on the shadow set are the same, including those blocks that are not currently allocated to files. System managers can use this command at their convenience during off-peak demands on their computing environment.

If the /ERASE qualifier is not used when the shadow set is created with the INITIALIZE/SHADOW command, and the SET SHADOW/DEMAND_MERGE DSAn: command is not executed, then the overhead of a full merge operation on this shadow set is even higher than is normally encountered after a system failure.

System managers can also use the SET SHADOW/DEMAND_MERGE DSAn: command for the following reasons:

  • If the ANALYZE/DISK/SHADOW command finds differences between the members of the shadow set.

  • If they want to measure the impact that a minimerge or a full merge has on their I/O throughput.