Re: unexpected NFS timeouts, related to sync/async soft mounts over TCP

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 10/11/11 20:43, Trond Myklebust wrote:
> On Thu, 2011-11-10 at 15:52 +0000, Andrew Cooper wrote: 
>> On 10/11/11 15:29, Chuck Lever wrote:
>>> On Nov 10, 2011, at 6:15 AM, Andrew Cooper wrote:
>>>
>>>> On 09/11/11 22:36, Chuck Lever wrote:
>>>>> On Nov 9, 2011, at 1:38 PM, Andrew Cooper wrote:
>> Sorry.  I am not sure I was clear.  An EIO does not present itself with
>> a hard mount, but a TCP FIN is still injected into the stream by the
>> client, causing 15 seconds of deadlock, eventually fixed by sending a
>> RST and restarting with a new TCP stream.  At this point, softmounts
>> throw an EIO while hardmounts restart and continue successfully.
>>
>> My problem is not the EIO on softmount or lack of EIO for hardmout, but
>> the fact that the client sees fit to try and close the TCP stream while
>> an apparently otherwise healthy NFS session is ongoing.
> The client will attempt to close the TCP connection on any RPC level
> error. That can happen, e.g., if the server sends a faulty RPC/TCP
> record fragment header or some other garbage data.
>
> I'm assuming that you've checked that the TCP parameters are set to sane
> values for a 10GigE connection (i.e. tcp_timestamps is on) so that there
> is no corruption happening at that level?
>
> Cheers
>   Trond

I have a TCPdump/wireshark analysis of the entire packet stream (4GiB). 
I cant see any RPC level errors (rpc.replystat != 0 yields no matches). 
What specifically would I be looking for?  Wireshark seems not to have
any problem decoding any of the RPC packets, so I hope that indicates no
RPC level corruption.

There is one case where the server sends a double write reply for the
same write, with different length fields.  However, this is a good 20
seconds before the FIN is sent, so I was hoping that it was unrelated.
Might it not be?

As for TCP timestamps; I have a Timestamp option in each TCP packet. 
Nothing appears corrupted.  What would I be looking for with corrupted
timestamps?

Thanks,

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com