Date: Wed, 21 Dec 2011 01:23:43 +0000 From: John De <jwd@freebsd.org> To: Rick Macklem <rmacklem@uoguelph.ca> Cc: freebsd-fs@freebsd.org Subject: Re: NFS client UDP retransmit timer busted for 8.n/9.n (patch) Message-ID: <20111221012343.GA86024@FreeBSD.org> In-Reply-To: <255844377.375232.1324256353832.JavaMail.root@erie.cs.uoguelph.ca> References: <255844377.375232.1324256353832.JavaMail.root@erie.cs.uoguelph.ca>
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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. -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.
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