Managing Serviceguard 14th Edition, June 2007

Designing Highly Available Cluster Applications
Designing Applications to Run on Multiple Systems
Appendix C 449
applications must move together. If the applications’ data stores are in
separate volume groups, they can switch to different nodes in the event
of a failover.
The application data should be set up on different disk drives and if
applicable, different mount points. The application should be designed to
allow for different disks and separate mount points. If possible, the
application should not assume a specific mount point.
To prevent one node from inadvertently accessing disks being used by
the application on another node, HA software uses an exclusive access
mechanism to enforce access by only one node at a time. This exclusive
access applies to a volume group as a whole.
Use Multiple Destinations for SNA Applications
SNA is point-to-point link-oriented; that is, the services cannot simply
be moved to another system, since that system has a different
point-to-point link which originates in the mainframe. Therefore, backup
links in a node and/or backup links in other nodes should be configured
so that SNA does not become a single point of failure. Note that only one
configuration for an SNA link can be active at a time. Therefore, backup
links that are used for other purposes should be reconfigured for the
primary mission-critical purpose upon failover.
Avoid File Locking
In an NFS environment, applications should avoid using file-locking
mechanisms, where the file to be locked is on an NFS Server. File locking
should be avoided in an application both on local and remote systems. If
local file locking is employed and the system fails, the system acting as
the backup system will not have any knowledge of the locks maintained
by the failed system. This may or may not cause problems when the
application restarts.
Remote file locking is the worst of the two situations, since the system
doing the locking may be the system that fails. Then, the lock might
never be released, and other parts of the application will be unable to
access that data. In an NFS environment, file locking can cause long
delays in case of NFS client system failure and might even delay the
failover itself.