NFS client UDP retransmit timer busted for 8.n/9.n (patch)

[Rick Macklem]

John De wrote:
> Hi Rick,
> 
> ----- Rick Macklem's Original Message -----
> > Thanks to recent detective work done by jwd@, a problem w.r.t.
> > retransmit timeouts for UDP mounts (both old and new NFS clients)
> > has been identified.
> >
> > The kernel rpc has two timeouts for UDP:
> > 1 - a timeout that causes the RPC request to be retransmitted on
> >     the same socket, using the same xid. This one defaults to
> >     3seconds and can be set via CLSET_RETRY_TIMEOUT.
> >     (This is always the default of 3seconds for FreeBSD currently.)
> > 2 - a timeout that cause the socket to be destroyed and a fresh
> >     one created. The request is then sent on this new socket, with
> >     a different xid.
> >
> > The problem with #2 is that the retransmitted RPC request will miss
> > a server's Duplicate Request Cache (DRC), because of the different
> > xid.
> > As such, #2 should be much larger than #1. However, #2 defaults to
> > 1second
> > (ie. smaller than #1->trouble!)
> >
> > One way to avoid this problem is to set #2 to a much larger value
> > via the
> > "timeout=<value>" mount option. (Btw, the <value> is in 1/10
> > seconds, so
> > "timeout="600" sets it to 60sec.)
> >
> > I now have a patch that I believe deals with this correctly. It sets
> > #1
> > to the "timeout=<value>" (default 1second) and #2 to a much larger
> > value.
> > (#2 timeouts are what the kernel rpc counts as retries, so for
> > "soft"
> >  mounts, I set #2 to "nm_retry * nm_timeout / 2" and "retries = 2",
> >  so
> >  that it fails after "nm_retry * nm_timeout", which I think is the
> >  correct
> >  semantics.)
> > This patch is attached and is also available at:
> >    http://people.freebsd.org/~rmacklem/udp-timer.patch
> > (jwd@, this patch is updated from what I emailed you, so you
> > probably want it:-)
> 
> We've tested both the mount_nfs option change and the patch and both
> seem to work great. No re-occurence of the problem we were seeing.
> We've only
> been able to catch one retransmit via tcpdump and it showed the system
> retransmitting with the same xid/port and recovering.
> 
> +1 for getting this committed.
> 
> Thanks for your time looking into this Rick.
> 
And thanks for reporting it. Also, sorry everyone, this has been broken
for a long time and I never spotted it.

rick

> -John
> 
> > In summary, if you are using NFS mounts over UDP on FreeBSD8 or 9
> > systems, you
> > either want to use "timeout=600" or try the patch. You are pretty
> > badly broken
> > otherwise.
> >
> > Hopefully, this patch can make it into -current/head soon, rick
> > ps: jhb@, could you maybe review this, thanks, rick.