From: Neil Horman <nhorman@redhat.com>
Subject: Re: rapid clustered nfs server failover and hung clients -- how best to	close the sockets?
Date: Mon, 9 Jun 2008 12:04:54 -0400
Message-ID: <20080609160454.GD20181@hmsendeavour.rdu.redhat.com>
References: <20080609103137.2474aabd@tleilax.poochiereds.net> <484D4659.9000105@redhat.com> <20080609152321.GA20181@hmsendeavour.rdu.redhat.com> <484D4E37.3060001@redhat.com>
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Cc: Neil Horman <nhorman@redhat.com>, Jeff Layton <jlayton@redhat.com>,
	linux-nfs@vger.kernel.org, nfsv4@linux-nfs.org, lhh@redhat.com
To: Peter Staubach <staubach@redhat.com>
In-Reply-To: <484D4E37.3060001@redhat.com>
Sender: linux-nfs-owner@vger.kernel.org

On Mon, Jun 09, 2008 at 11:37:27AM -0400, Peter Staubach wrote:
> Neil Horman wrote:
> >On Mon, Jun 09, 2008 at 11:03:53AM -0400, Peter Staubach wrote:
> >  
> >>Jeff Layton wrote:
> >>    
> >>>Apologies for the long email, but I ran into an interesting problem the
> >>>other day and am looking for some feedback on my general approach to
> >>>fixing it before I spend too much time on it:
> >>>
> >>>We (RH) have a cluster-suite product that some people use for making HA
> >>>NFS services. When our QA folks test this, they often will start up
> >>>some operations that do activity on an NFS mount from the cluster and
> >>>then rapidly do failovers between cluster machines and make sure
> >>>everything keeps moving along. The cluster is designed to not shut down
> >>>nfsd's when a failover occurs. nfsd's are considered a "shared
> >>>resource". It's possible that there could be multiple clustered
> >>>services for NFS-sharing, so when a failover occurs, we just manipulate
> >>>the exports table.
> >>>
> >>>The problem we've run into is that occasionally they fail over to the
> >>>alternate machine and then back very rapidly. Because nfsd's are not
> >>>shut down on failover, sockets are not closed. So what happens is
> >>>something like this on TCP mounts:
> >>>
> >>>- client has NFS mount from clustered NFS service on one server
> >>>
> >>>- service fails over, new server doesn't know anything about the
> >>> existing socket, so it sends a RST back to the client when data
> >>> comes in. Client closes connection and reopens it and does some
> >>> I/O on the socket.
> >>>
> >>>- service fails back to original server. The original socket there
> >>> is still open, but now the TCP sequence numbers are off. When
> >>> packets come into the server we end up with an ACK storm, and the
> >>> client hangs for a long time.
> >>>
> >>>Neil Horman did a good writeup of this problem here for those that
> >>>want the gory details:
> >>>
> >>>   https://bugzilla.redhat.com/show_bug.cgi?id=369991#c16
> >>>
> >>>I can think of 3 ways to fix this:
> >>>
> >>>1) Add something like the recently added "unlock_ip" interface that
> >>>was added for NLM. Maybe a "close_ip" that allows us to close all
> >>>nfsd sockets connected to a given local IP address. So clustering
> >>>software could do something like:
> >>>
> >>>   # echo 10.20.30.40 > /proc/fs/nfsd/close_ip
> >>>
> >>>...and make sure that all of the sockets are closed.
> >>>
> >>>2) just use the same "unlock_ip" interface and just have it also
> >>>close sockets in addition to dropping locks.
> >>>
> >>>3) have an nfsd close all non-listening connections when it gets a
> >>>certain signal (maybe SIGUSR1 or something). Connections on a
> >>>sockets that aren't failing over should just get a RST and would
> >>>reopen their connections.
> >>>
> >>>...my preference would probably be approach #1.
> >>>
> >>>I've only really done some rudimentary perusing of the code, so there
> >>>may be roadblocks with some of these approaches I haven't considered.
> >>>Does anyone have thoughts on the general problem or idea for a solution?
> >>>
> >>>The situation is a bit specific to failover testing -- most people 
> >>>failing
> >>>over don't do it so rapidly, but we'd still like to ensure that this
> >>>problem doesn't occur if someone does do it.
> >>>
> >>>Thanks,
> >>> 
> >>>      
> >>This doesn't sound like it would be an NFS specific situation.
> >>Why doesn't TCP handle this, without causing an ACK storm?
> >>
> >>    
> >
> >You're right, its not a problem specific to NFS, any TCP based service in 
> >which
> >sockets are not explicitly closed on the application are subject to this
> >problem.  however, I think NFS is currently the only clustered service 
> >that we
> >offer in which we explicitly leave nfsd running during such a 'soft' 
> >failover,
> >and so practically speaking, this is the only place that this issue 
> >manifests
> >itself.  If we could shut down nfsd on the server doing a failover, that 
> >would
> >solve this problem (as it prevents the problem with all other clustered tcp
> >based services), but from what I'm told, thats a non-starter.
> >
> >  
> 
> I think that this last would be a good thing to pursue anyway,
> or at least be able to understand why it would be considered to
> be a "non-starter".  When failing away a service, why not stop
> the service on the original node?
> 
> These floating virtual IP and ARP games can get tricky to handle
> in the boundary cases like this sort of one.
> 
> >As for why TCP doesnt handle this, thats because the situation is 
> >ambiguous from
> >the point of view of the client and server.  The write up in the bugzilla 
> >has
> >all the gory details, but the executive summary is that during rapid 
> >failover,
> >the client will ack some data to server A in the cluster, and some to 
> >server B
> >in the cluster.  If you quickly fail over and back between the servers in 
> >the
> >cluster, each server will see some gaps in the data stream sequence 
> >numbers, but
> >the client will see that all data has been acked.  This leaves the 
> >connection in
> >an unrecoverable state.
> 
> I would wonder what happens if we stick some other NFS/RPC/TCP/IP
> implementation into the situation.  I wonder if it would see and
> generate the same situation?
> 
>       ps
I can only imagine it would.  The problem doesn't stem from any particular
ideosyncracy in the provided nfsd, but rather in the fact that the nfsd is kept
running on both servers between failovers.

Neil


-- 
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 *Neil Horman
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