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
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