Silicon Graphics InfiniteStorage 4000 Series, Настройка и обслуживание

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144 How Synchronous Mirroring Works
record on the mirror repository volume. The controller then sends an I/O completion
indication back to the host system. Synchronous write mode is selected as the default
value and is the recommended write mode.
Asynchronous Write Mode
Asynchronous write mode offers faster host I/O performance but does not guarantee
that a copy operation has successfully completed before processing the next write
request. When you use Asynchronous write mode, host write requests are written to
the primary volume. The controller then sends an “I/O complete” indication back to
the host system, without acknowledging that the data has been successfully copied to
the secondary (remote) storage array.
When using Asynchronous write mode, write requests are not guaranteed to be
completed in the same order on the secondary volume as they are on the primary
volume. If the order of write requests is not retained, data on the secondary volume
might become inconsistent with the data on the primary volume. This event could
jeopardize any attempt to recover data if a disaster occurs on the primary storage
array.
Write Consistency Mode
When multiple mirror relationships exist on a single storage array and have been
configured to use Asynchronous write mode and to preserve consistent write order,
they are considered to be an interdependent group that is in the
Write consistency
mode
. The data on the secondary, remote storage array cannot be considered fully
synchronized until all of the remote mirrors that are in the Write consistency mode 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 arrays is prevented to protect the consistency of the remote data set.
Link Interruptions or
Secondary Volume
Errors
When processing write requests, the primary controller might be able to write to the
primary volume, 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 volume,
and the primary volume and the secondary volume are no longer correctly mirrored.
The primary controller transitions 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 volume, but remote writes do not take place.
When communication is restored between the controller owner of the primary volume
and the controller owner of the secondary volume, 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