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

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 this occurrence is extremely low, if the business cannot afford even a
minor amount of risk, then it is required to 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.
NOTE: Domino Mode is not supported in asynchronous mode.
• SRDF firmware has been configured and hardware has been installed on both Symmetrix
units.
• R1 and R2 devices must be correctly defined and assigned to the appropriate host systems in
the internal configuration that is downloaded by EMC.
• 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 of these states can cause the package to halt due to unexpected
conditions. In general, it is recommended that no manual change of states be performed 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. This group name is placed variable DEVICE_GROUP defined in the pkg.env file.
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 toolkit.
• To minimize contention, each device group used in the package should be assigned two
unique gatekeeper devices on the Symmetrix for each host where the package will run. These
gatekeeper devices must be associated with the Symmetrix device groups for that package.
The gatekeeper devices are typically a 2 MB logical device on the Symmetrix.
For example, if a package is configured to failover across four nodes in the cluster, there
should be eight gatekeeper devices (two for each node) that are assigned to the Symmetrix
device group belonging to this package.
Some Further Points 289