IBM DS8880 Series Introduction And Planning Manual page 86

Global Mirror
Practice copying and consistency groups
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DS8880 Introduction and Planning Guide
volume to guarantee that the source and target volume contain the
same data. When you establish volume pairs and choose 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 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.
Provides a long-distance remote copy across two sites by 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.
Global Mirror integrates both the Global Copy and FlashCopy functions.
The Global Mirror function mirrors data between volume pairs of two
storage systems over greater distances without affecting overall
performance. It also provides application-consistent data at a recovery (or
remote) site in 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 start 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 that is managed by the
master storage system, you can copy multiple volumes to the recovery site
simultaneously 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.)
Global Mirror supports up to 32 Global Mirror sessions per storage facility
image. Previously, only one session was supported per storage facility
image.
You can use multiple Global Mirror sessions to fail over only data assigned
to one host or application instead of forcing you to fail over all data if one
host or application fails. This process provides increased flexibility to
control the scope of a failover operation and to assign different options and
attributes to each session.
The DS CLI and DS Storage Manager display information about the
sessions, including the copy state of the sessions.
To get a consistent copy of your data, you can pause Global Mirror on a
consistency group boundary. Use the pause command with the secondary
storage option. (For more information, see the DS CLI Commands
reference.) After verifying that Global Mirror is paused on a consistency
boundary (state is Paused with Consistency), the secondary storage system
and the FlashCopy target storage system or device are consistent. You can
then issue either a FlashCopy or Global Copy command to make a practice
copy on another storage system or device. You can immediately resume
Global Mirror, without the need to wait for the practice copy operation to