ocfs2: Add a mount option "coherency=*" to handle cluster coherency for O_DIRECT writes.

Currently, the default behavior of O_DIRECT writes was allowing
concurrent writing among nodes to the same file, with no cluster
coherency guaranteed (no EX lock held).  This can leave stale data in
the cache for buffered reads on other nodes.

The new mount option introduce a chance to choose two different
behaviors for O_DIRECT writes:

    * coherency=full, as the default value, will disallow
                      concurrent O_DIRECT writes by taking
                      EX locks.

    * 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.

Signed-off-by: Tristan Ye <tristan.ye@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

1 files changed, 7 insertions, 0 deletions

diff --git a/Documentation/filesystems/ocfs2.txt b/Documentation/filesystems/ocfs2.txt
index 1f7ae144f6d8..5393e6611691 100644
--- a/Documentation/filesystems/ocfs2.txt
+++ b/Documentation/filesystems/ocfs2.txt
@@ -87,3 +87,10 @@ dir_resv_level=	(*)	By default, directory reservations will scale with file
                         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.