FreeBSD NFS client goes into infinite retry loop

[John Baldwin]

On Friday 19 March 2010 7:34:23 am Steve Polyack wrote:
> Hi, we use a FreeBSD 8-STABLE (from shortly after release) system as an 
> NFS server to provide user home directories which get mounted across a 
> few machines (all 6.3-RELEASE).  For the past few weeks we have been 
> running into problems where one particular client will go into an 
> infinite loop where it is repeatedly trying to write data which causes 
> the NFS server to return "reply ok 40 write ERROR: Input/output error 
> PRE: POST:".  This retry loop can cause between 20mbps and 500mbps of 
> constant traffic on our network, depending on the size of the data 
> associated with the failed write.
> 
> We spent some time on the issue and determined that something on one of 
> the clients is deleting a file as it is being written to by another NFS 
> client.  We were able to enable the NFS lockmgr and use lockf(1) to fix 
> most of these conditions, and the frequency of this problem has dropped 
> from once a night to once a week.  However, it's still a problem and we 
> can't necessarily force all of our users to "play nice" and use lockf/flock.
> 
> Has anyone seen this before?  No errors are being logged on the NFS 
> server itself, but the "Server Ret-Failed" counter begins to increase 
> rapidly whenever a client gets stuck in this infinite retry loop:
> Server Ret-Failed
>          224768961
> 
> I have a feeling that using NFS in such a matter may simply be prone to 
> such problems, but what confuses me is why the NFS client system is 
> infinitely retrying the write operation and causing itself so much grief.

Yes, your feeling is correct.  This sort of race is inherent to NFS if you do 
not use some sort of locking protocol to resolve the race.  The infinite 
retries sound like a client-side issue.  Have you been able to try a newer OS 
version on a client to see if it still causes the same behavior?

-- 
John Baldwin