Designing Disaster Recovery Clusters using Metroclusters and Continentalclusters, Reprinted October 2011 (5900-1881)

define it. All devices defined in a given device group must be configured with the same fence level.
A fence level of DATA or NEVER results in synchronous data replication; a fence level of ASYNC
is used to enable asynchronous data replication.
Fence Level of NEVER
Fence level = NEVER should only be used when the availability of the application is more important
than the data currency on the remote P9000 or XP disk arrays. In the case when all Continuous
Access links fail, the application will continue to modify the data on PVOL side, however the new
data is not replicated to the SVOL side. The SVOL only contains a copy of the data up to the point
of Continuous Access links failure. If an additional failure, such as a system failure before the
Continuous Access link is fixed, causes the application to fail over to the SVOL side, the application
will have to deal with non-current data.
If Fence level = NEVER is used, the data may be inconsistent in the case of a rolling
disaster—additional failures taking place before the system has completely recovered from a
previous failure. See an example of rolling disaster in the following section “Fence Level of DATA”.
Fence Level of DATA
Fence level = DATA is recommended to ensure a current and consistent copy of the data on all
sides. If Fence level = DATA is not enabled, the data may be inconsistent in the case of a rolling
disaster—additional failures taking place before the system has completely recovered from a
previous failure. Fence level = DATA is recommended, in case of Continuous Access link failure,
to ensure there is no possibility of inconsistent data at the SVOL side. Since only dedicated
Continuous Access links are supported, the probability of intermittent link failure and inconsistent
data at the remote (SVOL) side is extremely low. Additionally, if the following sequence of events
occur, it will cause inconsistent and therefore unusable data:
• Fence level = DATA is not enabled.
• The Continuous Access links fail.
• The application continues to modify data.
• The link is restored.
• Resynchronization from PVOL to SVOL starts, but does not finish.
• The PVOL side fails
Although the risk of this sequence of events taking place is extremely low, if your business cannot
afford a minimal level of risk, then enable Fence level = DATA to ensure that the data at the SVOL
side are always consistent.
The disadvantage of enabling Fence level = DATA is when the Continuous Access link fails, or if
the entire remote (SVOL) data center fails, all I/Os will be refused to the PVOL devices until the
Continuous Access link is restored, or manual intervention is used to split the PVOL side from the
SVOL side.
NOTE: Using manual intervention will allow the application to write the data to the PVOL side
without replicating the data to SVOL side. The data may be inconsistent in the case of a rolling
disaster. See the above example.
Applications may fail or may continuously retry the I/Os (depending on the application) if Fence
level = DATA is enabled and the Continuous Access link fails.
NOTE: If data currency is required on all sides, Fence level = DATA should be used and manual
intervention should NOT be taken when the Continuous Access links fail.
Overview of Continuous Access P9000 and XP Concepts 155