Designing Disaster Tolerant High Availability Clusters, 10th Edition, March 2003 (B7660-90013)

Building a Metropolitan Cluster Using MetroCluster/CA
Preparing an MC/ServiceGuard Cluster for MetroCluster /CA
Chapter 3 89
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 XP
disk array. In the case when all CA 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 CA links failure. If an additional failure, such as a
system failure before the CA 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 to ensure that there is no possibility
of inconsistent data at the SVOL side in case of CA link failure. Since
only dedicated CA links are supported, the probability of intermittent
link failure is extremely low. Therefore, the probability of inconsistent
data at the remote (SVOL) side is extremely low. However, inconsistent
and therefore unusable data will result from the following sequence of
circumstances:
• Fence level = DATA is not enabled.
• The CA 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