IBM DS8000 User Manual page 144

Session
Control path
Summary: How Global Mirror works
The automatic cycle in an active Global Mirror session works as follows to
maintain data at a remote site to be a point-in-time consistent copy of data at the
local site.
1. Consistency groups of volumes are created at the local site.
2. Increments of consistent data are sent to the remote site.
3. FlashCopy operations are performed at the remote site.
4. Global Copy operations are resumed between the local and remote site to copy
5. The steps are repeated according to the defined time intervals.
Fast restore operations:
The fast reverse option allows you to reverse a FlashCopy relationship without
waiting for the background copy of a previous FlashCopy to finish.
In disaster recovery situations, when you use Global Mirror, you must use volumes
with consistent data at the target site. Part of the setup requires that you enable the
fast restore option, a copy option that is available for FlashCopy operations. (This
option applies to a Global Mirror operation, which is based on existing Global
Copy and FlashCopy operations at the target site.)
Use fast restore option to reverse a FlashCopy target volume and allow consistent
data to be copied back to its associated source volume, without having to wait for
the background copy from the original source to the original target to complete.
You can then vary the volumes online and start your applications.
You can enable the fast reverse option by accessing Copy Services functions from
the Web interface called the IBM System Storage DS Storage Manager and
navigating to the FlashCopy page and then to the Select copy options page.
Global Mirror consistency groups:
Global Mirror can control the formation of consistency groups for data consistency.
Global Mirror is based on a combination of Global Copy and FlashCopy functions.
To support consistent data across storage units, Global Mirror uses a function
called sessions for forming consistent copies. A consistency group is a collection of
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DS8000 User's Guide
A session is a collection of volumes across multiple storage units that are
managed together to create consistent copies of data. The session is
identified with an ID that is unique across the enterprise. The ID identifies
volumes that will participate in the Global Mirror consistency group. A
session is opened on each LSS in the enterprise that has or might have
volumes that will participate in the Global Mirror consistency group that is
associated with the specific session ID.
The control path is established from the master storage unit to the
subordinate storage unit when more than one storage unit participates in
the Global Mirror session. If there is only one storage unit that is involved,
you do not have to create a control path. The master storage unit
communicates directly with its subordinate storage unit.
out of sync tracks.