IBM DS8800, Introduction And Planning Manual, Page: 82 / 256

With Metro Mirror, application write performance is dependent on the available
bandwidth. Global Copy allows you to better use your available bandwidth
capacity, therefore allowing you to include more of your data to be protected.
The enhancement to Global Copy is Global Mirror, which uses Global Copy and
the benefits of FlashCopy to form consistency groups. (A consistency group is a set
of volumes that contain consistent and current data to provide a true data backup
at a remote site.) Global Mirror uses a master storage unit (along with optional
subordinate storage units) to internally, without external automation software,
manage data consistency across volumes using consistency groups.
Consistency groups can also be created using the freeze and run functions of Metro
Mirror. The freeze and run functions, when used with external automation
software, provide data consistency for multiple Metro Mirror volume pairs.
The following sections describe the remote mirror and copy functions.
Synchronous mirroring (Metro Mirror)
Provides real-time mirroring of logical volumes (a source and a target)
between two storage units that can be located up to 300 km from each
other. With Metro Mirror copying, the source and target volumes can be on
the same storage unit or on separate storage units. You can locate the
storage unit at another site, some distance away.
Metro Mirror is a synchronous copy feature where write operations are
completed on both copies (local and remote site) before they are considered
to be complete. Synchronous mirroring means that a storage server
constantly updates a secondary copy of a volume to match changes made
to a source volume.
The advantage of synchronous mirroring is that there is minimal host
impact for performing the copy. The disadvantage is that since the copy
operation is synchronous, there can be an impact to application
performance because the application I/O operation is not acknowledged as
complete until the write to the target volume is also complete. The longer
the distance between primary and secondary storage units, the greater this
impact to application I/O, and therefore, application performance.
Asynchronous mirroring (Global Copy)
Copies data nonsynchronously and over longer distances than is possible
with the Metro Mirror feature. When operating in Global Copy mode, the
source volume sends a periodic, incremental copy of updated tracks to the
target volume instead of a constant stream of updates. This causes less
impact to application writes for source volumes and less demand for
bandwidth resources, while allowing a more flexible use of the available
bandwidth.
The updates are tracked and periodically copied to the target volumes. As
a consequence, there is no guarantee that data is transferred in the same
sequence that was applied to the source volume. To get a consistent copy
of your data at your remote site, you must periodically switch from Global
Copy to Metro Mirror mode, then either stop the application I/O or freeze
data to the source volumes using a manual process with freeze and run
commands. The freeze and run functions can be used with external
automation software such as Geographically Dispersed Parallel Sysplex
™
(GDPS), which is available for System z environments, to ensure data
consistency to multiple Metro Mirror volume pairs in a specified logical
subsystem.
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Introduction and Planning Guide