IBM System Storage DS3000, Programming Manual

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Global Mirror
When multiple mirror relationships exist on a single storage subsystem and have been configured to use
Global copy and to preserve consistent write order, they are considered to be an interdependent group
that is in the
Global mirror . The data on the secondary, remote storage subsystem cannot be considered
fully synchronized until all of the remote mirrors that are in the Global mirror are synchronized.
If one mirror relationship in the group becomes unsynchronized, all of the mirror relationships in the
group become unsynchronized. Any write activity to the remote, secondary storage subsystems is
prevented to protect the consistency of the remote data set.
Link Interruptions or Secondary Logical Drive Errors
When processing write requests, the primary controller might be able to write to the primary logical
drive, but a link interruption might prevent communication with the remote (secondary) controller.
In this case, the remote write operation cannot be completed to the secondary logical drive, and the
primary logical drive and the secondary logical drive are no longer correctly mirrored. The primary
controller migrations the mirrored pair into an Unsynchronized state and sends an I/O completion to the
primary host. The primary host can continue to write to the primary logical drive, but remote writes do
not take place.
When communication is restored between the controller owner of the primary logical drive and the
controller owner of the secondary logical drive, a resynchronization takes place. This resynchronization
happens automatically, or it must be started manually, depending on which write mode you chose when
setting up the mirror relationship. During the resynchronization, only the blocks of data that have
changed on the primary logical drive during the link interruption are copied to the secondary logical
drive. After the resynchronization starts, the mirrored pair migrations from an Unsynchronized status to a
Synchronization in Progress status.
The primary controller also marks the mirrored pair as unsynchronized when a logical drive error on the
secondary side prevents the remote write from completing. For example, an offline secondary logical
drive or a failed secondary logical drive can cause the remote mirror to become unsynchronized. When
the logical drive error is corrected (the secondary logical drive is placed online or recovered to an
Optimal status), then synchronization is required. The mirrored pair then migrations to a Synchronization
in Progress status.
Resynchronization
Data replication between the primary logical drive and the secondary logical drive in a mirror
relationship is managed by the controllers and is transparent to host machines and applications. When
the controller owner of the primary logical drive receives a write request from a host, the controller first
logs information about the write to a mirror repository logical drive. The controller then writes the data
to the primary logical drive. The controller then initiates a write operation to copy the affected data to the
secondary logical drive on the remote storage subsystem.
If a link interruption or a logical drive error prevents communication with the secondary storage
subsystem, the controller owner of the primary logical drive migrations the mirrored pair into an
Unsynchronized status. The controller owner then sends an I/O completion to the host sending the write
request. The host can continue to issue write requests to the primary logical drive, but remote writes to
the secondary logical drive do not take place.
When connectivity is restored between the controller owner of the primary logical drive and the
controller owner of the secondary logical drive, the logical drives must be resynchronized by copying the
blocks of data that changed during the interruption to the secondary logical drive. Only the blocks of
data that have changed on the primary logical drive during the link interruption are copied to the
secondary logical drive.
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IBM System Storage DS3000, DS4000, and DS5000: Command Line Interface and Script Commands Programming Guide