IBM DS8000 User Manual page 156

Periodically, the incremental resynchronization function performs queries from
each of the A volumes at the local site to each of the B volumes at the intermediate
site to determine when data has been secured on the C volumes at the remote site.
The queries generate a report that allows you to view the following types of
information:
v How often consistency groups are being formed in the Global Mirror
v The number of successful consistency groups that have formed since the last
In addition to the periodic queries, you can issue a query from the DS CLI
command prompt using the showgmir -metrics command. This command
generates a report that displays the status of consistency group formation.
If the Metro Mirror volume pairs are in full duplex status and at least three
consistency groups have formed successfully, then data from the local site volumes
is secure on the remote site volumes. The incremental resynchronization function
uses a mechanism to determine how often and how many tracks must change on
the specified primary volumes before it resets (and clears) the bitmaps after in
flight data is secured on the remote site volumes. This process continues until you
disable the incremental resynchronization function on the primary volumes using
the mkpprc command or the primary Metro Mirror volumes become secondary
volumes.
If you started the incremental resynchronization function with initialization and an
outage occurs at the intermediate site, you can restart a recovery configuration
with Global Mirror running between the local and remote sites.
Starting the incremental resynchronization function without
initialization
Starting incremental resynchronization without initialization means that the change
recording bitmaps are created but started with all zeros on the primary Metro
Mirror volumes. Do not specify the incremental resynchronization function without
initialization except in some specific recovery scenarios.
Recovering with the incremental resynchronization function
In an existing Metro/Global Mirror configuration, if the intermediate site fails, use
the mkpprc command with the -incrementalresync recover parameter to restart
your disaster recovery environment. This initiates the processing of Global Mirror
between the local and remote sites.
Before the mkpprc command completes its process, the system verifies that the
specified volumes have the intermediate volumes in common. This verification
ensures that the primary volume specified in the command is the Metro Mirror A
volume and the secondary volume specified is the Global Copy C volume of a
Metro/Global Mirror configuration.
When the mkpprc command processes, only changes on the A volumes that have
the change recording mechanism started are copied to the C volumes at the remote
site. A full volume copy of these volumes is not required.
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