From: Ulrich Gemkow <ulrich.gemkow@ikr.uni-stuttgart.de>
To: "J. Bruce Fields" <bfields@fieldses.org>
Subject: Re: NFS Server prevents access to files on different scenarios (lock problem?)
Date: Sun, 20 Nov 2016 12:33:02 +0100
Cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
References: <201611172132.47523.ulrich.gemkow@ikr.uni-stuttgart.de> <201611181955.51758.ulrich.gemkow@ikr.uni-stuttgart.de> <20161118204754.GG5424@fieldses.org>
In-Reply-To: <20161118204754.GG5424@fieldses.org>
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Hallo Bruce,

On Friday 18 November 2016, J. Bruce Fields wrote:
> On Fri, Nov 18, 2016 at 07:55:50PM +0100, Ulrich Gemkow wrote:
> > On Friday 18 November 2016, J. Bruce Fields wrote:
> > > On Thu, Nov 17, 2016 at 10:34:20PM +0100, Ulrich Gemkow wrote:
> > > > I found the 90sec lease time period but the timeout is sometimes
> > > > much longer than 90 sec, often up to 3minutes or longer. Is there
> > > > something which may cause these longer delays (I played with the
> > > > 90sec constant and it did not help :-)
> > > 
> > > A delegation is the only thing that I can think of that would prevent a
> > > file from being deleted on the server (by that you mean, not even a "rm
> > > blockfiled" run from a terminal on the server works?)  Delegations
> > > should definitely be forcibly revoked after the lease period passes.
> > > Note that you need to reboot (well, restart the nfs server) after
> > > changing the lease period, or the change will not take effect.
> > 
> > Thanks for this hint, I will disable delegations. But - the timeout
> > is for sure longer than 90 seconds in many cases. Can the reason be
> > a bad interaction between dropped tcp-connections (which may require
> > some time to be noticed) and the nfs server state(s)?
> 
> If the problem is a delegation, then what happens is essentially:
> 
> 	- you try to modify (or rename, or remove) the delated file.
> 	- the server sets a timer for the lease time (90s by default).
> 	- at the same time, the server notifies the client that it
> 	  should return the delegation.
> 	- if the timer expires then the server gives up and forcibly
> 	  removes the delegation, allowing your original operation to
> 	  proceed.
> 
> So tcp connections and stuff are details, what matters to the server is
> how much time has elapsed since you attempted an operation that
> conflicts with the delegation.  If that's significantly more than the
> lease period, then something's wrong.  So if you have a case where
> that's reliably too long, that would be interesting.

ok, thanks for the explanation, this helps a lot to understand
the correlations. I will disable delegations and see whether
something changed. I will come back then to this.

> > > > > For example, if the client held some delegations that it didn't return
> > > > > on unmount, and then it denied knowledge of them when the server tried
> > > > > to recall them, then the server would have to wait a lease period to
> > > > > forcibly remove them.  But, the client should be returning delegations
> > > > > on unmount, so I don't see how this happens.
> > > > > 
> > > > > For locks and opens and other state, again the client should be
> > > > > returning them on unmount.  And anyway the server isn't going to
> > > > > forcibly remove those ever, unless the entire client goes away
> > > > > completely, e.g. in a client crash or network partition.
> > > > > 
> > > > > So, I don't know.  Are you sure there aren't client crashes or network
> > > > > problems?
> > > > 
> > > > It happens that clients crash
> > > 
> > > I'm not sure what you mean there--do you mean clients are involved in
> > > all of these cases, or some of them?
> > 
> > Cause for the client reboots are impatient users which switch power
> > off-and-on when a hang happens. So the crashes (reboots) are not
> > directly related but the hangs happen often after such unwanted
> > reboots.
> 
> Hm.  So their stale state should be cleared out either 90 seconds after
> the client turned off, or as soon as the client comes back up and
> remounts, whichever comes first.  If that's not happening, again that
> sounds like a potentially interesting bug.

Currently I only see the delay without having a chance to further
debug this because I cannnot see the "inner state" of client and/or
server. I will try to use the debug messages to see more.

Thanks again and best regards

-Ulrich

> 
> --b.
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-- 
| Ulrich Gemkow
| University of Stuttgart
| Institute of Communication Networks and Computer Engineering (IKR)
|-----------------------------------------------------------------------