You can group Metro
Mirror or Global Mirror relationships
into a consistency group so that they can be updated at the same time.
A command that is issued to the consistency group is simultaneously
applied to all of the relationships in the group.
Metro
Mirror or Global Mirror relationships
can be based on "loose" or "tight" associations. A more
significant use arises when the relationships contain virtual disks
(VDisks) with a tight association. A simple example of a tight association
is the spread of data for an application across more than one VDisk.
A more complex example is when multiple applications run on different
host systems. Each application has data on different VDisks, and these
applications exchange data with each other. In both examples, specific
rules exist as to how the relationships can be updated. This ensures
that the set of secondary VDisks contain usable data. The key property
is that these relationships are consistent.
Metro
Mirror or Global Mirror relationships
can only belong to one consistency group; however, they do not have
to belong to a consistency group. Relationships that are not part
of a consistency group are called stand-alone relationships. A consistency
group can contain zero or more relationships. All relationships in
a consistency group must have matching primary and secondary clusters, which
are sometimes referred to as master and auxiliary clusters.
All relationships in a consistency group must also have the same copy
direction and state.
Metro
Mirror and
Global Mirror relationships
cannot belong to the same consistency group. A copy type is automatically
assigned to a consistency group when the first relationship is added
to the consistency group. After the consistency group is assigned
a copy type, only relationships of that copy type can be added to
the consistency group. Each cluster can have a maximum of six different
types of consistency groups. The following types of consistency groups
are possible:
- Intracluster Metro
Mirror
- Intercluster Metro
Mirror from
the local cluster to remote cluster
- Intercluster Metro
Mirror from
the remote cluster to local cluster
- Intracluster Global Mirror
- Intercluster Global Mirror from
the local cluster to remote cluster
- Intercluster Global Mirror from
the remote cluster to local cluster
States
A consistency group can be in one
of the following states:
- Inconsistent (stopped)
- The primary VDisks are accessible for read and write I/O operations
but the secondary VDisks are not accessible for either. A copy process
must be started to make the secondary VDisks consistent.
- Inconsistent (copying)
- The primary VDisks are accessible for read and write I/O operations
but the secondary VDisk are not accessible for either. This state
is entered after the svctask startrcconsistgrp command is issued
to a consistency group in the InconsistentStopped state. This state
is also entered when the svctask startrcconsistgrp command
is issued, with the force option, to a consistency group in the Idling
or ConsistentStopped state.
- Consistent (stopped)
- The secondary VDisks contain a consistent image, but it might
be out-of-date with respect to the primary VDisks. This state can
occur when a relationship was in the ConsistentSynchronized state
and experiences an error that forces a freeze of the consistency group.
This state can also occur when a relationship is created with the
CreateConsistentFlag set to TRUE.
- Consistent (synchronized)
- The primary VDisks are accessible for read and write I/O operations.
The secondary VDisks are accessible for read-only I/O operations.
- Idling
- Both the primary VDisks and the secondary VDisks are operating
in the primary role. Consequently the VDisks are accessible for write
I/O operations.
- Idling (disconnected)
- The VDisks in this half of the consistency group are all operating
in the primary role and can accept read or write I/O operations.
- Inconsistent (disconnected)
- The VDisks in this half of the consistency group are all operating
in the secondary role and cannot accept read or write I/O operations.
- Consistent (disconnected)
- The VDisks in this half of the consistency group are all operating
in the secondary role and can accept read I/O operations but not write
I/O operations.
- Empty
- The consistency group does not contain any relationships.