Unexplained NFS mount hangs

[Daniel Stickney <dstickney@pronto.com>]

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

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