reservations - users should rarely need to change this
value. If allocation reservations are turned off, this
option will have no effect.
+coherency=full (*) Disallow concurrent O_DIRECT writes, cluster inode
+ lock will be taken to force other nodes drop cache,
+ therefore full cluster coherency is guaranteed even
+ for O_DIRECT writes.
+coherency=buffered Allow concurrent O_DIRECT writes without EX lock among
+ nodes, which gains high performance at risk of getting
+ stale data on other nodes.