Data corruption over NFS in -current

Dan Nelson dnelson at
Thu Jan 12 06:06:08 UTC 2012

In the last episode (Jan 11), Martin Cracauer said:
> Rick Macklem wrote on Wed, Jan 11, 2012 at 08:42:25PM -0500: 
> > Also, if you can reproduce the problem fairly easily, capture a packet
> > trace via
> > # tcpdump -s 0 -w xxx host <server> 
> > running on the client (or similar). Then email me "xxx" as an attachment
> > and I can look at it in wireshark.  (If you choose to look at it in
> > wireshark, I would suggest you look for Create RPCs to see if they are
> > Exclusive Creates, plus try and see where the data for the corrupt file
> > is written.)
> > 
> > Even if the capture is pretty large, it should be easy to find the
> > interesting part, so long as you know the name of the corrupt file and
> > search for that.
> That's probably not practical, we are talking about hammering the NFS
> server with several CPU hours worth of parallel activity in a shellscript
> but I'll do my best :-)

The tcpdump options -C and -W can help here.  For example, -C 1000 -W 10
will keep the most recent 10-GB of traffic by circularly writing to 10 1-GB
capture files.  All you need to do is kill the tcpdump when you discover the
corruption, and work backwards through the logs until you find your file.

	Dan Nelson
	dnelson at

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