GFS is a far far better choice these days for a clustering file system. We've run sessions stored on GFS and never had the locking issues Sam described with NFS. We also have our Moodledata on GFS.
With GFS, watch out for it's issues with slow access for small-files, like those used in Sessions. For this reason a better option is simply to store the sessions in the database, Moodle has this as a default option. On PGSQL or MySQL this would perform very well. On Oracle it has it's issues due to the CLOB data field, but nothing that can't be managed.
We have been intending to test Memcache sessions, but we have had no real need to actually investigate it as yet as storing the sessions in the Database has been near-perfect.
Hardware and performance
Sessions on memcached?
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