On Tue, 12 May 2009, Robert Watson wrote:
Normally, NFS clients implement open-to-close consistency, which dictates
that when a close() occurs on client A, all pending writes on the file
should be issued to the server before close() returns, so that a signal to
client B to open() the file can validate its cache before open() returns.
This should, of course, read "close-to-open consistency" -- I plead jetlag
after an overnight flight back form Boston to the UK :-)
Robert N M Watson
Computer Laboratory
University of Cambridge
This raises the following question: is client A closing the file, and is
client B then opening it?
If not: relying on writes being visible on the client B before the close() on
A and a fresh open() on B is not guaranteed to work, although we can discuss
ways to improve behavior with respect to expectation. Try modifying your
application and see if it gets the desired behavior, and then we can discuss
ways to improve what you're seeing.
If you are: this is probably a bug in our caching and or issuing of NFS RPCs.
We cache both attribute and access data -- perhaps there is an open() path
where we issue neither RPC? In the case of open, we likely should test for a
valid access cache entry, and if there is one, issue an attribute read, and
otherwise just issue an access check which will piggyback fresh attribute
data on the reply. Perhaps there is a bug here somewhere.
A few other misc questions:
- Could you confirm you're using NFSv3 on all clients. Are there any special
mount options in use?
- What version of FreeBSD are you running with?
In FreeBSD 8.x, we now have DTrace probes for all of the above events --
VOPs, attribute cache hit/miss/load/flush, access cache hit/miss/load/flush,
RPCs, etc, which we can use to debug the problem. I haven't yet MFC'd these
to 7.x, but if you're able to run a very fresh 7-STABLE, I can probably
produce a patch to add it for you in a few days.
Robert N M Watson
Computer Laboratory
University of Cambridge
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