NFS directory listing hang on write.

[Russell Miller]

Hi, all.  We're having some NFS problems here that I don't understand.

The arguments are standard.  wsize=32768,rsize=32768,tcp.  Nothing
else out of the ordinary.  Test case is easy too:

mount a filesystem.
cd into the directory.
create a 1G file and background the creation, so 1G is being written
into the directory.
Then, try to ls the directory.

The ls will hang, sometimes for a few seconds, sometimes for the
entire length of the write.  This happens on every NFS server I've
tried, including an acopia, bluearc, onstor, and a generic linux box
running on the other end.

I have seen this behavior on multiple systems and using multiple
kernels.  Starting with the latest centos 5.2 kernel, but I've also
reproduced it on the stock 2.6.27.7.  I don't really understand what's
going on, but I think it has something to do with the attribute cache,
as setting actimeo=1 seems to have a positive effect.  Setting it to
zero seems to have no effect.

It does not seem to be happening with centos 4.x kernels, which are
based on 2.6.9.

I have turned on nfs and rpc debugging, and it's not telling me
anything useful, which is what I would expect if the attribute cache
is borking somehow.  I don't think there's any debugging for the cache
itself.

Can someone please give me an idea of how to proceed in debugging
this?  I'm not a stranger to the kernel, but this is deeper in than
I've gone before, and I am frankly a little over my head here.

Thanks,
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