Building Disaster Recovery Serviceguard Solutions Using Metrocluster with EMC SRDF

• The Domino Mode must be enabled for M x N configuration to ensure the following:
data currency on all Symmetrix frames.◦
◦ no possibility of inconsistent data at the R2 side in case of SRDF links failure.
If Domino Mode is not enabled and all SRDF links fail, the new data is not replicated to the
R2 side while the application continues to modify the data on the R1 side. This results in the
R2 side containing a copy of the data only up to the point of the SRDF link failure. If additional
failure occurs, such as a system failure, before the SRDF link is fixed, the application fails over
to the R2 side with non-current data.
If Domino Mode is not enabled, in the case of a rolling disaster, the data may be inconsistent.
Additional failures may take place before the system has completely recovered from a previous
failure. 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 the data.
◦ the link is restored.
◦ resynchronization from R1 to R2 starts, but does not finish.
◦ the R1 side fails.
Although the risk of such an occurrence is extremely low, if the business cannot afford even
a minor risk, then the Domino Mode must be enabled to ensure that the data at the R2 side
is always consistent.
The disadvantage of enabling Domino Mode is that when the SRDF link fails, applications
might fail or continuously retry I/Os as I/Os are refused until the SRDF link is restored, or
manual intervention is undertaken to disable Domino Mode.
NOTE: Domino Mode is not supported in asynchronous mode.
• SRDF firmware must be configured and hardware installed on both Symmetrix units.
• While the cluster is running, all Symmetrix devices that belong to the same Serviceguard
package, and defined in a single SRDF device group must be in the same state at the same
time. Manual changes to these states can cause the package to halt due to unexpected
conditions. HP recommends that you do not manually change the states while the package
and the cluster are running.
• A single Symmetrix device group must be defined for each package on each host that is
connected to the Symmetrix. The disk special device file names for all Volume Groups that
belong to the package must be defined in one Symmetrix device group for both R1 side and
R2 side.
The Symmetrix device group name must be the same on each host for both R1 side and R2
side.
Although the name of the device group must be the same on each node, the special device
file names specified may be different on each node.
Symmetrix Logical Device names must be default names of the form “DEVnnn” (for example,
DEV001). Do not use the option for creating your own device names.
See the EMC Solutions Enabler manual, and the sample convenience scripts in the Samples
directory included with this product.
36 Configuring an application in a Metrocluster solution