Silicon Graphics InfiniteStorage 4000 Series, Настройка и обслуживание

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Chapter 7: Using the Asynchronous Mirroring Premium Feature 123
The controller owner of the primary volume initiates remote writes to the secondary
volume to keep the data on the two volumes synchronized.
The secondary volume maintains a mirror (or copy) of the data on its associated
primary volume. The controller owner of the secondary volume receives remote
writes from the controller owner of the primary volume but does not accept host write
requests. Hosts are able to read from the secondary volume, which appears as
read-only.
In the event of a disaster or a catastrophic failure at the primary site, you can perform
a role reversal to promote the secondary volume to a primary role. Hosts then are able
to read from and write to the newly promoted volume, and business operations can
continue.
When write operations are performed to the primary volume of an asynchronous
mirrored pair, the modified data region of the primary volume is tracked. Periodically,
the firmware creates a new point-in-time image of the primary volume and sends the
changed data regions to the secondary volume. When data synchronization completes,
the system uses the point-in-time images on the secondary volume to ensure that the
data is maintained in a consistent state during subsequent synchronization operations
to the secondary volume.
The automatic cycle in an active asynchronous mirroring session works as follows to
maintain data at a remote site to be a point-in-time consistent copy of data at the local
site.
1. Asynchronous mirror groups consisting of volumes are created at the local site.
2. Increments of consistent data are sent to the remote site.
3. Point-in-time copy operations are performed at the remote site.
These steps are repeated according to the defined mirroring intervals.
The fundamental assumptions of the Asynchronous Mirroring premium feature are
these:

The data link between the primary storage array and the secondary storage array
has significantly lower bandwidth and higher latency than the host interface. For
this reason, the mirrored data movement must be decoupled from the primary
host data requests to minimize the performance impact to the host application.
Additionally, with a restricted data link unsynchronized regions between the
primary and secondary volumes must be tracked in sufficiently small granularity
to minimize data movement. Long distance data links to remote storage arrays
require additional networking equipment, which can result in varying throughput
performance.