IBM DS8000 - ADDITIONAL INFORMATION, User Manual, Page: 146 / 250

A Copy Services session is a collection of volumes across multiple storage units
that are managed together to create consistent copies of data. Sessions are managed
for Global Mirror relationships.
The Global Mirror mode supports data consistency across multiple volumes,
logical subsystems (LSSs), and storage units by using a concept called sessions. For
disaster recovery, it is required that the volumes in a session that are necessary for
recovery be consistent up to the time of failure. If data on these volumes is copied
out of sequence, then the recovery procedures that use these volumes become
useless. Global Mirror uses special algorithms to provide data consistency.
Sessions are associated Global Mirror relationships and are defined with an
identifier (session ID) that is unique across the enterprise. The ID identifies
volumes that are related and that can participate in the Global Mirror consistency
group.
You can select a set of volumes across LSSs on multiple storage units to participate
in consistency groups. Logical volumes within a given LSS can be part of different
sessions and different consistency groups. After you create a session, you can add
logical volumes to or remove logical volumes from, a particular session. After a set
of logical volumes become part of the session, a single session-based
″
command
″
can operate on all of the logical volumes in the session.
Metro/Global Mirror
The Metro/Global Mirror function combines the capabilities of Metro Mirror and
Global Mirror functions for greater protection against planned and unplanned
outages.
Metro/Global Mirror is a three-site, high availability disaster recovery solution,
which uses synchronous replication to mirror data between a local site and an
intermediate site, and asynchronous replication to mirror data from an
intermediate site to a remote site. DS8000 storage units support the Metro/Global
Mirror function on open systems and System z or S/390 hosts. You can set up and
manage your Metro/Global Mirror configurations using DS CLI and TSO
commands.
In a Metro/Global Mirror configuration, a Metro Mirror volume pair is established
between two nearby sites (local and intermediate) to protect from local site
disasters. The Global Mirror volumes can be located thousands of miles away and
can be updated if the original local site has suffered a disaster but has performed
failover operations to the intermediate site. In the case of a local-site-only disaster,
Metro/Global Mirror can provide zero-data-loss recovery at the remote site as well
as at the intermediate site.
In some customer environments, it is necessary to mirror data from a local to a
remote site within the distance that is supported for synchronous mirroring. This is
especially true when synchronous I/O is required for high or near continuous
availability and when a zero-data-loss configuration is required. However, in some
cases, it is desirable to have more than a short distance synchronous mirroring
solution. Sometimes the following mirroring solutions are required:
v
A nearby two-site synchronous copy that can protect from local disasters.
v A longer distance asynchronous copy, at a third site, that can protect from larger
scale regional disasters. The third site provides an extra layer of data protection.
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DS8000 User’s Guide