On Aug 28, 2008, at Aug 28, 2008, 1:41 PM, Tom Tucker wrote:
Paolo Galtieri wrote:
Folks,
I have a question regarding the behavior of nfsd v3 running over
TCP. Specifically how should the NFS server behave if the clock is
moved backwards on the server. I've tried to find references to
what the correct behavior should be and have been unable to, so I'm
posting to this list with the hope someone can point me to where I
can find it.
I'm running a 2.6.10 kernel on 2 systems, one acting as an NFS
server the other as the client (obviously). I advertise to the
client a directory that contains several files which I mount on the
client as /app
On the server I run the following test:
while ;
do
date 022910002008;
sleep 10;
date 022910102008;
sleep 10;
done
on the client I run the following in /app
while :
do
ls
sleep 10
done
What I observe is that often I see long pauses when doing the ls
before it actually displays the data.
I ran tcpdump and what I observed is that the NFS server closed the
connection when the timestamp of ACK for a packet sent by the
client is earlier than the timestamp for the original packet, i.e.
between the receipt of the original packet and the sending of the
ACK the clock went backwards. For example:
Both the client and server have logic that shuts down idle
connections. The client idle time out is 5m and the server's idle
timeout is 6m. What is probably happening is that the server
believes that the connection has been idle longer than the 6m
timeout and shuts down the connection.
I don't know for sure, but it seems like the server's network layer
would drop a connection if it thought it was getting ACKs way out of
order. It may be a defense against malicious behavior.
If the client detects that the server dropped the connection, it does
wait a bit before reconnecting. This prevents a hard loop where the
server drops, the client immediately reconnects, the server drops
again, the client reconnects, and so on.
There are some cases where the server drops because it's got a
listener, but it's not actually ready to take NFS requests yet; or,
the server drops a connection because it is overloaded; or, the server
still has a listener, but is being shut down.
The reason the client waits only 5 minutes for an idle timeout is to
try to ensure it will disconnect first. Otherwise, if the server
idles out a connection first, the client will see this and wait before
reconnecting when there is more work to do.
The client-side logic to wait before reconnecting has seen some
evolution during 2.6. So it hasn't always worked quite correctly and
has seen some changes over time. It would be interesting if you tried
this test with later kernels.
Frame 1019 contains this packet
TCP nfs > legent-1 [FIN, ACK] Seq=961 Ack=937 Win=1448 Len=0
TSV=979162569 TSER=979004710
0000 00 14 f8 ec 30 2f 00 14 f8 ec 47 11 08 00 45 00 ....
0/.. ..G...E.
0010 00 34 2a 2c 40 00 40 06 ba 95 a9 fe 01 01 a9 fe .
4*,@.@. ........
0020 01 05 08 01 01 75 03 a6 6d a7 a9 d3 d4 ff 80 11 .....u..
m.......
0030 05 a8 69 d4 00 00 01 01 08 0a 3a 5c d5 c9 3a
5a ..i..... ..:\..:Z
0040 6d 26
and is time stamped (using wireshark) 2008-02-29 02:00:00.911651.
The original packet for which this is the ACK is frame 924 and
contains:
TCP legent-1 > nfs [ACK] Seq=937 Ack=961 Win=1460 Len=0
TSV=979004710 TSER=979159895
0000 00 14 f8 ec 47 11 00 14 f8 ec 30 2f 08 00 45 00 ....G... ..
0/..E.
0010 00 34 2b e9 40 00 40 06 b8 d8 a9 fe 01 05 a9 fe .
4+.@.@. ........
0020 01 01 01 75 08 01 a9 d3 d4 ff 03 a6 6d a7 80
10 ...u.... ....m...
0030 05 b4 74 3b 00 00 01 01 08 0a 3a 5a 6d 26 3a
5c ..t;.... ..:Zm&:\
0040 cb
57 .W
Wireshark reports the RTT as:
[The RTT to ACK the segment was: -7.377099000 seconds]
After the server disconnects the client has to reconnect and it is
looks like the delay is occurring during this disconnect/reconnect
process.
Can someone point me to any documentation, either for NFS or TCP
that explains the behavior? Note that this problem does not occur
with UDP so I suspect it's a consequence of the connection oriented
aspects of the TCP protocol vs UDP, but it would nice to see it in
writing.
I appreciate any assistance, flames, or other comments :-)
Thank you,
Paolo
--
Chuck Lever
chuck[dot]lever[at]oracle[dot]com
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