IBM DS8000 - ADDITIONAL INFORMATION, User Manual, Page: 56 / 250

same data. When you establish volume pairs and elect not to copy
a volume, a relationship is established between the volumes but no
data is sent from the source volume to the target volume. In this
case, it is assumed that the volumes contain exactly the same data
and are consistent, so copying the entire volume is not necessary
or required. Only new updates are copied from the source to target
volumes.
Global Mirror
Provides a long-distance remote copy across two sites using asynchronous
technology. Global Mirror processing is most often associated with disaster
recovery or disaster recovery testing. However, it can also be used for
everyday processing and data migration.
The Global Mirror function mirrors data between volume pairs of two
storage units over greater distances without affecting overall performance.
It also provides application-consistent data at a recovery (or remote) site in
case of a disaster at the local site. By creating a set of remote volumes
every few seconds, the data at the remote site is maintained to be a
point-in-time consistent copy of the data at the local site.
Global Mirror operations periodically invoke point-in-time FlashCopy
operations at the recovery site, at regular intervals, without disrupting the
I/O to the source volume, thus giving a continuous, near up-to-date data
backup. By grouping many volumes into a session, which is managed by
the master storage unit, you can copy multiple volumes to the recovery
site simultaneously while maintaining point-in-time consistency across
those volumes. (A session contains a group of source volumes that are
mirrored asynchronously to provide a consistent copy of data at the remote
site. Sessions are associated with Global Mirror relationships and are
defined with an identifier [session ID] that is unique across the enterprise.
The ID identifies the group of volumes in a session that are related and
that can participate in the Global Mirror consistency group.)
Metro/Global Mirror
Provides a three-site, long distance disaster recovery replication that
combines Metro Mirror with Global Mirror replication for both System z
and open systems data. Metro/Global Mirror 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.
In a three-site, Metro/Global Mirror, should an outage occur, a backup site
is maintained regardless of which one of the sites is lost. Suppose an
outage occurs at the local site, Global Mirror continues to mirror updates
between the intermediate and remote sites, maintaining the recovery
capability at the remote site. If an outage occurs at the intermediate site,
data at the local storage unit is not affected. If an outage occurs at the
remote site, data at the local and intermediate sites is not affected.
Applications continue to run normally in either case.
With the incremental resynchronization function enabled on a
Metro/Global Mirror configuration, should the intermediate site be lost,
the local and remote sites can be connected, and only a subset of changed
data is copied between the volumes at the two sites. This reduces the
amount of data that needs to be copied from the local site to the remote
site and the time it takes to do the copy.
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DS8000 User’s Guide