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

Physical Data Replication for ContinentalClusters Using EMC SRDF
Maintaining the EMC SRDF Data Replication Environment
Chapter 7 357
NOTE It is highly recommended that the R2 device is locally protected with
RAID 1 or RAID S. If the R2 device is protected with BCV, and if it
fails and there is a failover, the package cannot operate on the BCV
device. The R2 device has to be fixed, the data has to be restored
from the BCV device to the new R2 device, before the package can
start.
• 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