IBM DS8880 Series Introduction And Planning Manual page 85

between two storage systems 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 system or on separate storage systems. You can locate the
storage system 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 that
are 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 systems, 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 function causes
less impact to application writes for source volumes and less demand for
bandwidth resources. It allows for 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, periodically
switch from Global Copy to Metro Mirror mode, then either stop the
application I/O or freeze data to the source volumes by 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
environments, to ensure data consistency to multiple Metro Mirror volume
pairs in a specified logical subsystem.
Common options for Metro Mirror/Global 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/Global 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 system tracks
all changed data on the source volume, you can resume operations
later to synchronize the data between the volumes.
Copy out-of-synchronous data
You can specify that only data updated on the source volume
while the volume pair was suspended is 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
™ ®
(GDPS ), which is available for IBM Z
Chapter 3. Data management features
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