Silicon Graphics InfiniteStorage 4000 Series, Configuring And Maintaining

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Chapter 8: Using the Synchronous Mirroring Premium Feature 145
primary volume during the link interruption are copied to the secondary volume.
After the resynchronization starts, the mirrored pair transitions from an
Unsynchronized status to a Synchronization in Progress status.
The primary controller also marks the mirrored pair as unsynchronized when a
volume error on the secondary side prevents the remote write from completing. For
example, an offline secondary volume or a failed secondary volume can cause the
remote mirror to become unsynchronized. When the volume error is corrected (the
secondary volume is placed online or recovered to an Optimal status), then
synchronization is required. The mirrored pair then transitions to a Synchronization in
Progress status.
Resynchronization
Data replication between the primary volume and the secondary volume in a mirror
relationship is managed by the controllers and is transparent to host machines and
applications. When the controller owner of the primary volume receives a write
request from a host, the controller first logs information about the write to a mirror
repository volume. The controller then writes the data to the primary volume. The
controller then initiates a write operation to copy the affected data to the secondary
volume on the remote storage array.
If a link interruption or a volume error prevents communication with the secondary
storage array, the controller owner of the primary volume transitions 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 volume, but remote writes to the secondary volume do not take place.
When connectivity is restored between the controller owner of the primary volume
and the controller owner of the secondary volume, the volumes must be
resynchronized by copying the blocks of data that changed during the interruption to
the secondary volume. Only the blocks of data that have changed on the primary
volume during the link interruption are copied to the secondary volume.
ATTENTION Possible loss of data access
– Any communication disruptions
between the primary storage array and the secondary storage array while
resynchronization is underway could result in a mix of new data and old data on the
secondary volume. This condition would render the data unusable in a disaster
recovery situation.
Creating a
Synchronous
Mirroring Pair Before you create any mirror relationships, volumes must exist at both the primary
site and the secondary site. The volume that resides on the local storage array is the
primary volume. Similarly, the volume that resides on the remote storage array is the
secondary volume. If neither the primary volume nor the secondary volume exist, you
must create these volumes. Keep these guidelines in mind when you create the
secondary volume: