IBM DS8000 User Manual page 55

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.
Common options for Metro Mirror and Global Copy include the following
modes:
Suspend and resume
If you schedule a planned outage to perform maintenance at your
remote site, you can suspend Metro Mirror or Global Copy
processing on specific volume pairs during the duration of the
outage. During this time, data is no longer copied to the target
volumes. Because the primary storage unit keeps track of all
changed data on the source volume, you can resume operations at
a later time to synchronize the data between the volumes.
Copy out-of-synchronous data
You can specify that only data that was updated on the source
volume while the volume pair was suspended be copied to its
associated target volume.
Copy an entire volume or not copy the volume
You can copy an entire source volume to its associated target
volume to guarantee that the source and target volume contain the
Chapter 1. Introduction to IBM System Storage DS8000 series
™
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