Re: Unexplained NFS mount hangs

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On Apr 13, 2009, at 12:47 PM, Daniel Stickney wrote:

On Mon, 13 Apr 2009 12:12:47 -0400
Chuck Lever <chuck.lever@xxxxxxxxxx> wrote:

On Apr 13, 2009, at 11:24 AM, Daniel Stickney wrote:
Hi all,

I am investigating some NFS mount hangs that we have started to see
over the past month on some of our servers. The behavior is that the
client mount hangs and needs to be manually unmounted (forcefully
with 'umount -f') and remounted to make it work. There are about 85
clients mounting a partition over NFS. About 50 of the clients are
running Fedora Core 3 with kernel 2.6.11-1.27_FC3smp. Not one of
these 50 has ever had this mount hang. The other 35 are CentOS 5.2
with kernel 2.6.27 which was compiled from source. The mount hangs
are inconsistent and so far I don't know how to trigger them on
demand. The timing of the hangs as noted by the timestamp in /var/
log/messages varies. Not all of the 35 CentOS clients have their
mounts hang at the same time, and the NFS server continues operating
apparently normally for all other clients. Normally maybe 5 clients
have a mount hang per week, on different days, mostly different
times. Now and then we might see a cluster of a few clien
ts have their mounts hang at the same exact time, but this is not
consistent. In /var/log/messages we see

Apr 12 02:04:12 worker120 kernel: nfs: server broker101 not
responding, still trying

Are these NFS/UDP or NFS/TCP mounts?

If you use a different kernel (say, 2.6.26) on the CentOS systems, do
the hangs go away?

Hi Chuck,

Thanks for your reply. The mounts are NFSv3 over TCP. We have not tried a different kernel (because of the number of servers to be upgraded), but that is next on to ToDo list. Wanted to explore the possibility that some other change might resolve the issue, but I am getting close to launching the kernel upgrades. (The prepackaged RHEL/CentOS 2.6.18* kernels have other NFS client problems with attribute caching which really mess things up, so that is why we have had to compile from source)

To add a little more info, in a post on April 10th titled "NFSv3 Client Timeout on 2.6.27" Bryan mentioned that his client socket was in state FIN_WAIT2, and server in CLOSE_WAIT, which is exactly what I am seeing here.

tcp 0 0 worker120.cluster:944 broker101.cluster:nfs FIN_WAIT2

This is especially interesting because the original nfs "server not responding" message was about 32 hours ago. On this same client, all other NFS mounts to other servers are showing state "established".

Poking around in git, I see this recent commit:

commit 2a9e1cfa23fb62da37739af81127dab5af095d99
Author: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>
Date:   Tue Oct 28 15:21:39 2008 -0400

    SUNRPC: Respond promptly to server TCP resets

If the server sends us an RST error while we're in the TCP_ESTABLISHED state, then that will not result in a state change, and so the RPC client
    ends up hanging forever (see

We can intercept the reset by setting up an sk->sk_error_report callback,
    which will then allow us to initiate a proper shutdown and retry...

We also make sure that if the send request receives an ECONNRESET, then we
    shutdown too...

    Signed-off-by: Trond Myklebust <Trond.Myklebust@xxxxxxxxxx>

Which may address part of the problem. If I'm reading the output of "git describe" correctly, this one should be in 2.6.28.

There are a whole series of commits in this area that went upstream about a month ago. It's not clear if these are also necessary to address the problem. But they would be in 2.6.30-rc1.


One very interesting aspect of this behavior is that the load value
on the client with the hung mount immediately spikes to (16.00)+
(normal load value). We have also seen client load spikes to (30.00)+
(normal load value). These discrete load value increases might be a
good hint.

Running 'df' prints some output and then hangs when it reaches the
hung mount point. 'mount -v' shows the mount point like normal. When
an NFS server is rebooted, we are used to seeing the client log a
"nfs: server ___________ not responding, still trying", then a "nfs:
server __________ OK" message when it comes back online. With this
issue there is never an "OK" message even though the NFS server is
still functioning for all other NFS clients. On a client which has a
hung NFS mount, running 'rpcinfo -p' and 'showmount -e' against the
NFS server shows that RPC and NFS appear to be functioning between
client and server even during the issue.

# rpcinfo -p broker101
 program vers proto   port
  100000    2   tcp    111  portmapper
  100000    2   udp    111  portmapper
  100021    1   udp  32779  nlockmgr
  100021    3   udp  32779  nlockmgr
  100021    4   udp  32779  nlockmgr
  100021    1   tcp  60389  nlockmgr
  100021    3   tcp  60389  nlockmgr
  100021    4   tcp  60389  nlockmgr
  100011    1   udp    960  rquotad
  100011    2   udp    960  rquotad
  100011    1   tcp    963  rquotad
  100011    2   tcp    963  rquotad
  100003    2   udp   2049  nfs
  100003    3   udp   2049  nfs
  100003    4   udp   2049  nfs
  100003    2   tcp   2049  nfs
  100003    3   tcp   2049  nfs
  100003    4   tcp   2049  nfs
  100005    1   udp    995  mountd
  100005    1   tcp    998  mountd
  100005    2   udp    995  mountd
  100005    2   tcp    998  mountd
  100005    3   udp    995  mountd
  100005    3   tcp    998  mountd

# showmount -e broker101
Export list for broker101:
/mnt/sdc1 *
/mnt/sdb1 *

It is confusing that the NFS client doesn't recover automatically.
So whatever the issue is evidently is blocking the kernel from
seeing that the NFS server is live and functioning after the issue
is triggered.

I'm running low on ideas of how to resolve this. One idea I have is
to modify some NFS client timeout values, but I don't have a
specific reason to think this will resolve the problem. Right now
the values are:

# sysctl -a | grep -i nfs
fs.nfs.nlm_grace_period = 0
fs.nfs.nlm_timeout = 10
fs.nfs.nlm_udpport = 0
fs.nfs.nlm_tcpport = 0
fs.nfs.nsm_use_hostnames = 0
fs.nfs.nsm_local_state = 0
fs.nfs.nfs_callback_tcpport = 0
fs.nfs.idmap_cache_timeout = 600
fs.nfs.nfs_mountpoint_timeout = 500
fs.nfs.nfs_congestion_kb = 65152
sunrpc.nfs_debug = 0
sunrpc.nfsd_debug = 0

I've turned on nfs debugging but there was a tremendous amount of
output because of the NFS clients activity on several different (and
working) NFS mount points. I can capture and supply this output
again if it would be helpful. Has anyone seen this behavior before,
and does anyone have any suggestions for how this might be resolved?

Thanks for your time,

Daniel Stickney
Operations Manager - Systems and Network Engineer
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Daniel Stickney
Operations Manager - Systems and Network Engineer

Chuck Lever

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