IBM TotalStorage DS6000 Introduction And Planning Manual page 28

Maximum time writes are inhibited to the remote site
Session management:
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.
Consistency groups
Consistency groups are used for controlling error situations and for maintaining data
consistency at a recovery site.
You can create paths from a source LSS to a target LSS have the consistency
group option enabled. All volume pairs in remote mirror and copy relationships from
this source to target LSS (sharing the same paths) belong to the consistency group.
Other volumes pairs are not affected.
With the consistency group option enabled, when an error occurs on any volume
pairs or on the links that are associated with these LSS pairs, an alert is issued and
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Introduction and Planning Guide
Indicates the maximum time (in seconds) that write operations are not
allowed at the recovery site before the storage unit stops forming a current
consistency group. If the drain time is maintained for an extended period of
time, the number of write operations that are required to transfer the data to
the recovery site can become large enough to increase the time to form a
consistency group.
Note: As distance increases, there is an increasing lag in the time it takes
for the data to be written to the recovery site. This lag is referred to
as a drain time.
Loss of data can also be increased in the event of a disaster. If it is set to
zero, it defaults to either four minutes or twice the consistency group
interval value, depending on which value is larger. The first consistency
group is formed regardless of the consistency group drain time. For the
remainder of the consistency groups, if the specified time expires before the
data is drained to the recovery site, consistency group formation stops.
After the consistency group formation has stopped five consecutive times,
the timer is disabled and the next consistency group is formed regardless of
the required time.