536 Implementing the IBM Storwize V7000 with IBM Spectrum Virtualize V8.2.1
Connected versus disconnected
Under certain error scenarios (for example, a power failure at one site that causes one 
complete system to disappear), communications between two systems in an MM/GM 
relationship can be lost. Alternatively, the fabric connection between the two systems might 
fail, which leaves the two systems that are running but cannot communicate with each other.
When the two systems can communicate, the systems and the relationships that spans them 
are described as connected. When they cannot communicate, the systems and the 
relationships spanning them are described as disconnected.
In this state, both systems are left with fragmented relationships and are limited regarding the 
configuration commands that can be performed. The disconnected relationships are 
portrayed as having a changed state. The new states describe what is known about the 
relationship and the configuration commands that are permitted. 
When the systems can communicate again, the relationships are reconnected. MM/GM 
automatically reconciles the two state fragments, considering any configuration or other event 
that occurred while the relationship was disconnected. As a result, the relationship can return 
to the state that it was in when it became disconnected, or it can enter a new state. 
Relationships that are configured between volumes in the same IBM Storwize V7000 system 
(intracluster) are never described as being in a disconnected state.
Consistent versus inconsistent
Relationships that contain volumes that are operating as secondaries can be described as 
being consistent or inconsistent. Consistency Groups that contain relationships can also be 
described as being consistent or inconsistent. The consistent or inconsistent property 
describes the relationship of the data on the auxiliary to the data on the master volume. It can 
be considered a property of the auxiliary volume.
An auxiliary volume is described as consistent if it contains data that can be read by a host 
system from the master if power failed at an imaginary point while I/O was in progress, and 
power was later restored. This imaginary point is defined as the recovery point. 
The requirements for consistency are expressed regarding activity at the master up to the 
recovery point. The auxiliary volume contains the data from all of the writes to the master for 
which the host received successful completion and that data was not overwritten by a 
subsequent write (before the recovery point).
Consider writes for which the host did not receive a successful completion (that is, it received 
bad completion or no completion at all). If the host then performed a read from the master of 
that data that returned successful completion and no later write was sent (before the recovery 
point), the auxiliary contains the same data as the data that was returned by the read from 
the master. 
From the point of view of an application, consistency means that an auxiliary volume contains 
the same data as the master volume at the recovery point (the time at which the imaginary 
power failure occurred). If an application is designed to cope with an unexpected power 
failure, this assurance of consistency means that the application can use the auxiliary and 
begin operation as though it was restarted after the hypothetical power failure. Again, 
maintaining the application write ordering is the key property of consistency. 
For more information about dependent writes, see 11.1.13, “FlashCopy and image mode 
Volumes” on page 459.