HP Serviceguard Metrocluster with EMC SRDF for Linux B.01.00.00

Managing Business Continuity Volumes (BCV)
The use of BCV is recommended with all implementations of Metrocluster with EMC SRDF for Linux,
and it is required with M by N configurations, which employ consistency groups. These BCV
devices provide a good copy of the data when it is necessary to recover from a rolling disaster—a
second failure that occurs while attempting to recover from the first failure.
Protecting against rolling disasters
The following is an example of a rolling disaster with Metrocluster with EMC SRDF for Linux.
At time T0, all the SRDF links go down. The application continues to run on the R1 side. At time
T1, the SRDF links are restored, and at T2 a manual resynchronization is started to resync new
data from the R1 to the R2 side. At time T3, while resynchronization is in progress, the R1 site
fails, and the application starts up on the R2 side. The resynchronization did not complete when
there was a failure on the R1 side. Therefore, the data on the R2 side is corrupt.
Using the BCV in resynchronization
In the case described above, you use the business continuity volumes, which protect against a
rolling disaster. First split off a consistent copy of the data at the recovery site, and then perform
the re-synchronization. After the re-synchronization is complete, re-establish the BCV mirroring. To
protect data consistency on R2 in rolling disaster, use the following procedure:
1. Before starting the re-synchronization from R1 to R2 side, it is necessary to disable the package
switch capability to prevent the package automatically fail over to R2 if a new disaster occurs
when the re-sync is still in progress. Disable the package switching on the R2 nodes.
# cmmodpkg -d <package Name> -n <Node Name>
2. Split the BCV in the secondary Symmetrix from the mirror group to save a good copy of the
data from the nodes on the R2 side.
# symmir -g <Device Group Name> split
Alternatively, from the node on R1 side.
# symmir -g <Device Group Name> split -rdf
3. Begin to resynchronize the data from R1 to R2 devices.
# symrdf -g <Device Group Name> establish
4. After the resynchronization is completed, enable the package switching on the node on R2
side.
# cmmodpkg -e <package Name> -n <Node Name>
5. Re-establish the BCV to R2 devices on R2 as a mirror.
# symmir -g <Device Group Name> -full establish
Alternatively, from the node on R1 side.
# symmir -g <Device Group Name> -full establish-rdf
In Metrocluster with EMC SRDF for Linux environment, following the resynchronization process
described above, which prevents the package from automatically failing over and starting on the
R2 side if a disaster takes place when the resync is in progress. This ensures the package would
not automatically start and operate on the inconsistent data in the event of a rolling disaster.
As demonstrated above, the re-sync is a manual process and initiated by an operator after the
links are fixed. The pairstate of the devices must be Synchronized for SRDF/Synchronous or
Consistent for SRDF/Asynchronous when the re-sync is completed. Verify the state and ensure that
the re-sync is completed before enabling the package switch.
Managing Business Continuity Volumes (BCV) 35