The only 2 POSIX file-locking clustered file systems worth considering are GFS2 and OCFS2.
But be aware you will probably need to dedicate one core for locking overhead, since all it does is eat and serve data, as it is still a very CPU intensive task. Probably a better idea to use a script with keepalived on the secondary node pointing at the primary node, to fail over to an identically configured Splunk instance on the secondary node. Wont get load balancing between two active pair, but not eat a core just to do it in a cluster either.
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