On 25/09/14 02:44, NeilBrown wrote:
On Wed, 24 Sep 2014 16:39:55 +0100 Benjamin ESTRABAUD <be@xxxxxxxxxx> wrote:
Hi!
I've got a scenario where I'm connected to a NFS share on a client, have
a file descriptor open as read only (could also be write) on a file from
that share, and I'm suddenly changing the IP address of that client.
Obviously, the NFS share will hang, so if I now try to read the file
descriptor I've got open (here in Python), the "read" call will also hang.
However, the driver seems to attempt to do something (maybe
save/determine whether the existing connection can be saved) and then,
after about 20 minutes the driver transparently reconnects to the NFS
share (which is what I wanted anyways) and the "read" call instantiated
earlier simply finishes (I don't even have to re-open the file again or
even call "read" again).
The dmesg prints I get are as follow:
[ 4424.500380] nfs: server 10.0.2.17 not responding, still trying <--
changed IP address and started reading the file.
[ 4451.560467] nfs: server 10.0.2.17 OK <--- The NFS share was
reconnected, the "read" call completes successfully.
The difference between these timestamps is 27 seconds, which is a lot less
than the "20 minutes" that you quote. That seems odd.
Hi Neil,
My bad, I had made several attempts and must have copied the wrong dmesg
trace. The above happened when I manually reverted the IP config back to
its original address (when doing so the driver reconnects immediately).
Here is what had happened:
[ 1663.940406] nfs: server 10.0.2.17 not responding, still trying
[ 2712.480325] nfs: server 10.0.2.17 OK
If you adjust
/proc/sys/net/ipv4/tcp_retries2
you can reduce the current timeout.
See Documentation/networking/ip-sysctl.txt for details on the setting.
https://www.kernel.org/doc/Documentation/networking/ip-sysctl.txt
It claims the default gives an effective timeout of 924 seconds or about 15
minutes.
I just tried and the timeout was 1047 seconds. This is probably the next
retry after 924 seconds.
If I reduce tcp_retries2 to '3' (well below the recommended minimum) I get
a timeout of 5 seconds.
You can possibly find a suitable number that isn't too small...
That's very interesting! Thank you very much! However, I'm a bit worried
when changing the whole TCP stack settings, NFS is only one small chunk
of a much bigger network storage box, so if there are alternative it'll
probably be better. Also I would need a very very small timeout, in the
order of 10-20 secs *max* so that would probably cause other issues
elsewhere, but this is very interesting indeed.
Alternately you could use NFSv4. It will close the connection on a timeout.
In the default config I measure a 78 second timeout, which is probably more
acceptable. This number would respond to the timeo mount option.
If I set that to 100, I get a 28 second timeout.
This is great! I had no idea, I will definitely roll NFSv4 and try that.
Thanks again for your help!
The same effect could be provided for NFSv3 by setting:
__set_bit(NFS_CS_DISCRTRY, &clp->cl_flags);
somewhere appropriate. I wonder why that isn't being done for v3 already...
Probably some subtle protocol difference.
If for some reason we can't stick to v4 we'll try that too, thanks.
NeilBrown
Regards,
Ben - MPSTOR.
I would like to know if there was any way to tune this behaviour,
telling the NFS driver to reconnect if a share is unavailable after say
10 seconds.
I tried the following options without any success:
retry=0; hard/soft; timeo=3; retrans=1; bg/fg
I am running on a custom distro (homemade embedded distro, not based on
anything in particular) running stock kernel 3.10.18 compiled for i686.
Would anyone know what I could do to force NFS into reconnecting a
seemingly "dead" session sooner?
Thanks in advance for your help.
Regards,
Ben - MPSTOR.
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