In an OpenVMS Cluster system,
a master write bitmap is created
on the node that issues the DISMOUNT or MOUNT command that creates
the write bitmap. When a master write bitmap is created, a local write 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 write 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 write 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 write 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
write bitmap, the node sends a message for each write directly to
the master write bitmap. This will degrade application write performance.
HP Volume Shadowing for OpenVMS
Using Minicopy for Backing Up Data (Alpha)
