IBM DS8000 User Manual page 155

Mirror configuration. The volumes are continually updated to maintain a record of
tracks that have changed data that has not been secured at the remote site. If the
intermediate site fails, the change recording bitmaps and out-of-synchronization
bitmaps on the primary Metro Mirror volumes contain the tracks of data that are
needed to resynchronize the local site and remote site volumes. The
resynchronization process copies only a subset of changed data that has to be
copied between the local site and remote site volumes and eventually to the D
volumes (the FlashCopy target volumes in Global Mirror).
The incremental resynchronization function uses a change recording mechanism to
track in flight changes. Because there can only be one function that actively uses
change recording on a volume at a time, the incremental resynchronization
function prevents change recording for other purposes, such as incremental
FlashCopy processing. If an attempt is made to start incremental resynchronization
when another function using change recording is active, that start request fails. If
an attempt is made to do an incremental FlashCopy from a remote mirror and
copy primary volume that has the incremental resynchronization active, that
FlashCopy request fails.
When the intermediate site is operational, you can return to the original cascading
Metro/ Global Mirror configuration. You can restore your Metro/Global Mirror
configuration without interrupting production.
How Incremental Resynchronization works
If you enable the incremental resynchronization function for Metro/Global Mirror
and there is an intermediate site failure, the procedure to restart the recovery
environment running Global Mirror from the local site to the remote site is
simplified, and a full volume copy is not required.
The following sections describe starting the incremental resynchronization function
with initialization in a Metro/Global Mirror configuration and, in some very
specific situations, without initialization. Information is also provided about other
options that you can use with the incremental resynchronization function.
For best management practices, it is recommended that you manage your
Metro/Global Mirror configurations using external software in conjunction with
the IBM DS CLI or IBM TotalStorage Replication Manager.
Starting the incremental resynchronization function with initialization
Starting incremental resynchronization with initialization means that the change
recording mechanism is started on the specified primary Metro Mirror (A) volumes
in a Metro/Global Mirror configuration. This creates an environment on the
primary Metro Mirror volumes that enables the microcode to monitor and track
data "in flight" in a Metro/Global Mirror configuration. (In flight data refers to data
that has been written to the primary volumes but has not been secured on the C
volumes at the remote site.)
When you set up your Metro/Global Mirror configuration, you must enable the
incremental resynchronization function with the initialization option to create and
start change recording bitmaps on each of the primary Metro Mirror volumes.
When data is written to the primary volumes, an out-of-synchronization change
recording bitmap is also created on the primary Metro Mirror volumes, which is
later used for resynchronization between the local site and remote site volumes.
127
Chapter 8. Optional feature codes for the DS8000