IBM DS8000 User Manual page 150

A three-site Metro/Global Mirror configuration provides the following recovery
options at the alternate sites if a failure occurs:
v If an outage occurs at the local site, recovery operations can begin at the
v If an outage occurs at the local site, recovery operations can begin at the remote
v If an outage occurs at the intermediate site, data at the local storage unit is not
v If an outage occurs at the remote site, data at the local and intermediate sites is
v If both the local and intermediate sites are lost, the scenario is similar to a
Consistency group formation in a Metro/Global Mirror
environment
Consistency groups in a Metro/Global Mirror environment create a consistent
point-in-time copy of multiple volumes, with negligible host impact.
If there is a disaster at the local site, data consistency must be maintained across
multiple volumes. This requires ensuring that consistency groups form successfully
at the remote site and are valid to use. (A consistency group keeps data consistent
across multiple volumes in multiple storage units.)
There are two methods of maintaining consistent data in a Metro/Global Mirror
configuration:
Metro Mirror
Global Mirror
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intermediate site. Global Mirror continues to mirror updates between the
intermediate and remote sites, maintaining the recovery capability at the remote
site.
site and preparations can be made to resynchronize the local site when it
recovers from its disaster. Once in recovery mode at the remote site, another
Global Mirror session can be set up and put into operation using the former
intermediate as its new remote site. This new Global Mirror session provides
additional disaster recovery while operating at the remote site.
affected. Applications continue to run normally.
not affected. Applications continue to run normally. The intermediate storage
unit maintains a consistent up-to-date copy.
two-site scenario when access to the local storage unit is lost. Recovery must be
achieved using the last consistent point-in-time copy at the remote site.
The freeze function that is part of Metro Mirror operations provides data
consistency for a Metro Mirror environment. The freeze function can be
used by external automation software to ensure data consistency to
multiple Metro Mirror volume pairs in a specified LSS. External
automation software can detect messages that are generated by Metro
Mirror processing errors. You can temporarily suspend write operations to
the volume pairs that are associated with the specified LSS. This ensures a
point of consistency at the remote site, across all necessary volumes in the
specified LSSs. Afterwards, you can resume host I/O on the volumes pairs.
The integrated consistency capabilities of Global Mirror, which uses a
master storage unit (along with optional subordinate storage units)
internally manage data consistency using consistency groups in a Global
Mirror environment.
Note: The consistency of the Global Mirror replication depends on the
consistency of the Metro Mirror target volumes. Otherwise, even
though the Global Mirror target volumes will be a valid copy of the
Global Mirror source volumes during the mirroring process, they
might not necessarily be a valid copy of the production volumes.