In an OpenVMS Cluster system, a master bitmap is created on the node that issues the DISMOUNT or MOUNT command that creates the bitmap. When a master bitmap is created, a local bitmap is automatically created on all other nodes in the cluster on which the shadow set is mounted, provided the nodes have sufficient memory.

A master bitmap contains a record of all the writes to the shadow set from every node in the cluster that has the shadow set mounted. A local bitmap tracks all the writes that the local node issues to a shadow set.

Note that if a node with a local bitmap writes to the same logical block number (LBN) of a shadow set more than once, only the LBN of the first write is sent to the master bitmap. The minicopy operation uses the LBN for the update, not the number of changes to the same LBN.

When there is not enough memory on a node to create a local bitmap, the node sends a message for each write directly to the master bitmap. This degrades application write performance.