Chapter 11. Advanced Copy Services 519
Consider the following points:
/SM590000MM/GM relationships can be part of a consistency group, or they can be stand-alone and, 
therefore, are handled as single instances.
/SM590000A consistency group can contain zero or more relationships. An empty consistency group 
with zero relationships in it has little purpose until it is assigned its first relationship, except 
that it has a name.
/SM590000All relationships in a consistency group must have corresponding master and auxiliary 
volumes.
/SM590000All relationships in one consistency group must be the same type, for example only Metro 
Mirror or only Global Mirror.
Although consistency groups can be used to manipulate sets of relationships that do not need 
to satisfy these strict rules, this manipulation can lead to undesired side effects. The rules 
behind a consistency group mean that certain configuration commands are prohibited. These 
configuration commands are not prohibited if the relationship is not part of a consistency 
group.
For example, consider the case of two applications that are independent, yet they are placed 
into a single consistency group. If an error occurs, synchronization is lost and a background 
copy process is required to recover synchronization. While this process is progressing, 
MM/GM rejects attempts to enable access to the auxiliary volumes of either application.
If one application finishes its background copy more quickly than the other application, 
MM/GM still refuses to grant access to its auxiliary volumes even though it is safe in this case. 
The MM/GM policy is to refuse access to the entire consistency group if any part of it is 
inconsistent. Stand-alone relationships and consistency groups share a common 
configuration and state model. All of the relationships in a non-empty consistency group have 
the same state as the consistency group.
11.6.4  Remote copy intercluster communication 
In the traditional Fibre Channel, the intercluster communication between systems in a 
MM/GM partnership is performed over the SAN. This section describes this communication 
path. 
For more information about intercluster communication between systems in an IP 
partnership, see 11.8.6, “States of IP partnership” on page 554.
Zoning
At least two FC ports of every node of each system must communicate with each other to 
create the partnership. Switch zoning is critical to facilitate intercluster communication. 
Intercluster communication channels 
When an IBM Spectrum Virtualize system partnership is defined on a pair of systems, the 
following intercluster communication channels are established:
/SM590000A single control channel, which is used to exchange and coordinate configuration 
information
/SM590000I/O channels between each of these nodes in the systems
These channels are maintained and updated as nodes and links appear and disappear from 
the fabric, and are repaired to maintain operation where possible. If communication between 
the systems is interrupted or lost, an event is logged (and the MM/GM relationships stop).