Re: Question regarding CIFS cache=loose behavior.

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On Fri, 21 Mar 2014 05:56:04 +0900
Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> wrote:

> Jeff Layton wrote:
> > That's expected behavior. The kernel believes that the file is frozen in
> > length so it returns short read() calls until the size is updated.
> 
> The "size is updated" means "stat() detects the growth of file size",
> doesn't it? Then, the former is expected behavior.
> 
> > 
> > cache=loose is very much not recommended for use when you have multiple
> > hosts accessing files on the server (or access by processes on the
> > server itself). It only gives you "loose" cache coherency. The whole
> > point of it is to allow the client to cache data even when the protocol
> > says that it shouldn't.
> 
> But why is the latter ( "read() returns non-0 when stat() detects the growth
> of file size but the data actually read is '\0'" ) is expected behavior?
> It sounds like a bug that the client caches '\0' (data nobody has ever wrote)
> instead of '.' (data somebody wrote when the file size grew).
> 

Yeah, that sounds wrong. What should happen is that the cache is
invalidated when the size changes. It's possible there is a race in
that code however. The locking around it is pretty sloppy...

> > 
> > cache=strict is what is recommended and that's the default these days.
> 
> Yes, I suggested a customer using CIFS on RHEL6 to use cache=none or
> cache=strict , for that customer's system does not work as expected due to
> the latter behavior when using "tail -f" for checking for appended log.

Yeah, cache=strict should work much better for that sort of use-case.

-- 
Jeff Layton <jlayton@xxxxxxxxxx>
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