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

Building a Metropolitan Cluster Using MetroCluster/SRDF
Preparing a Cluster for MetroCluster/SRDF
Chapter 4 141
• Only Synchronous Mode is supported; Adaptive Copy must be
disabled.
• Domino Mode is recommended to ensure data currency on all
Symmetrix frames and that there is no possibility of inconsistent
data at the R2 side in case of SRDF links failure.
If Domino Mode is not enabled and if all SRDF links fail, and the
application continues to modify the data on the R1, but the new data
is not replicated to the R2 side. The R2 only contains a copy of the
data up to the point of CA links failure. If additional failure occurs,
such as a system failure before the SRDF link is fixed, this can cause
the application to fail over to the R2 side, and the application will
have to deal with non-current data.
If Domino Mode is not enabled, in the case of a rolling disaster, the
data may be inconsistent. Additional failures taking place before the
system has completely recovered from a previous failure. The
inconsistent and therefore unusable data will result from the
following sequence of circumstances:
— Domino Mode is not enabled
— the SRDF links fail
— the application continues to modify data
— The link is restored
— Resynchronization from R1 to R2 starts, but does not finish
— The R1 side fails
Although the risk of this occurrence is extremely low, if your
business cannot afford even this quite small risk, then you must
enable Domino Mode to ensure that the data at the R2 side are
always consistent. The disadvantage of enabling Domino Mode is
that when the SRDF link fails, all I/Os will be refused (to those
devices) until the SRDF link is restored, or manual intervention is
undertaken to disable Domino Mode. Applications may fail or may
continuously retry the I/Os (depending on the application) if Domino
Mode is enabled and the SRDF link fails.
• To minimize contention, each package should be assigned its own,
unique gatekeeper device. See “Configuring Gatekeeper Devices” on
page 155.