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.