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.