21.2 Enabling High Availability Features
High availability allows Moab to run on two different machines: a primary and secondary server. The configuration method to achieve this behavior takes advantage of a networked file system to configure two Moab servers with only one operating at a time.
When configured to run on a networked file system—any networked file system that supports file locking is supported—the first Moab server that starts locks a particular file. The second Moab server waits on that lock and only begins scheduling when it gains control of the lock on the file. This method achieves near instantaneous turnover between failures and eliminates the need for two Moab servers to synchronize information periodically as the two Moab servers access the same database/checkpoint file.
Because the two Moab servers access the same files, configuration is only required in the moab.cfg file. The two hosts that run Moab must be configured with the SERVER and FBSERVER parameters. File lock is turned on using the FLAGS=filelockha parameter. Finally, the lock file is specifiled with the HALOCKFILE parameter. The following example illustrates a possible configuration:
SCHEDCFG[Moab] SERVER=host1:42559 SCHEDCFG[Moab] FBSERVER=host2 SCHEDCFG[Moab] FLAGS=filelockha SCHEDCFG[Moab] HALOCKFILE=/opt/moab/.moab_lock
Adminstrators can run the mdiag -S -v command to view which Moab server is currently scheduling and responding to client requests.
Moab has many features to improve the availability of a cluster beyond the ability to automatically relocate to another execution server. The following table describes some of these features.
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