Re: [NFS] Cache flushing

[Timo Sirainen <tss@iki.fi>]

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On Tue, 2007-11-20 at 18:47 -0500, Trond Myklebust wrote:
> On Tue, 2007-11-20 at 04:14 +0200, Timo Sirainen wrote:
> > On Sun, 2007-11-18 at 00:11 +0200, Timo Sirainen wrote:
> > > > Why can't you simply close(), and then re-open() the file? That is _the_
> > > > standard way to force an attribute cache revalidation on all NFS
> > > > versions. The close-to-open caching model, which is implemented on most
> > > > NFS clients guarantees this.
> > ..
> > > close()+open() would have been difficult to handle because open() can
> > > fail, but looks like opening another file descriptor and closing it
> > > works just as well. Also looks like it works for flushing directories'
> > > attribute cache (which doesn't seem to work with FreeBSD though).
> > 
> > Actually it works for flushing a directory's attribute cache in
> > v2.6.17-rc2, but not in v2.6.22. chown() works in v2.6.22 also. Is there
> > a reason for this change? I guess it anyway means that I'm back to using
> > chown() for flushing a directory's attribute cache.
> 
> close-to-open caching works fine for me in v2.6.22. It does indeed send
> a GETATTR and revalidate the inode.

I don't have my own NFS test setup, so I can't check what actually
happens in the network, but I can easily reproduce this as a user:

1. touch foo.1 foo.2

2. Run on NFS client 1:

rm -f foo;ln foo.1 foo;sleep 0.5;rm -f foo;ln foo.2 foo

3. Run on NFS client 2 within that 0.5 secs:

echo *>/dev/null;stat foo;sleep 0.5;echo * >/dev/null;stat foo

After repeating this a few (2-5) times stat will return foo.1's inode
for both stats and running the command over and over again will return
only foo.1's inode. The only way to get NFS client 2 to notice the
change and return foo.2's inode is to either wait for attribute cache to
timeout or to run "chown 0 ."

Now that I think of it, I guess the reason is that my 2.6.22 setup has
Linux+ext3 as NFS server and 2.6.17-rc2 setup has NetApp as server. So
even though open()+close() flushes the directory's attribute cache, it
doesn't flush the file name -> NFS handle cache unless mtime changes?

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