580 Implementing the IBM Storwize V5000 Gen2 with IBM Spectrum Virtualize V8.1
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 V5000 Gen2
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 might 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 (app), 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.
If a relationship (or set of relationships) is inconsistent and an attempt is made to start an
application by using the data in the secondaries, the following outcomes are possible:
The app might decide that the data is corrupted and crash or exit with an event code.
The application might fail to detect that the data is corrupted and return erroneous data.
The application might work without a problem.
Because of the risk of data corruption, and in particular undetected data corruption, MM/GM
strongly enforces the concept of consistency and prohibits access to inconsistent data.
Consistency as a concept can be applied to a single relationship or a set of relationships in a
Consistency Group. Write ordering is a concept that an application can maintain across
several disks that are accessed through multiple systems. Therefore, consistency must
operate across all of those disks.
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