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

[Rick Macklem]

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:-)

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.