[Ocfs2-devel] [PATCH 1/1] Ocfs2: Add a mount option "coherency=*" to handle cluster coherency for O_DIRECT writes.

[Joel Becker]

On Tue, Oct 12, 2010 at 06:01:11AM +0800, Tao Ma wrote:
> Joel Becker wrote:
> >On Mon, Oct 11, 2010 at 04:46:39PM +0800, Tristan Ye wrote:
> >>Currently, default behavior of O_DIRECT writes was allowing
> >>concurrent writing among nodes, no cluster coherency guaranteed
> >>(no EX locks was taken), it hurts buffered reads on other nodes
> >>by reading stale data from cache.
> >>
> >>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 at oracle.com>
> >
> >	This patch is now in the merge-window branch of ocfs2.git.
> I think that you agree with me that we only need to take PR lock in
> full_coherency, but this patch
> still try the exclusive one. So do I miss something?

	I don't agree with you.  The PR does not force other nodes who
have a PR to drop their caches.  It does force them to write out, but
that isn't the whole problem.  We came to this because other nodes are
holding stale data from their reads, and we need them to drop that stale
data when we O_DIRECT write.

Joel

-- 

Life's Little Instruction Book #24

	"Drink champagne for no reason at all."

Joel Becker
Consulting Software Developer
Oracle
E-mail: joel.becker at oracle.com
Phone: (650) 506-8127