Re: NFS auto-reconnect tuning.

[NeilBrown <neilb@suse.de>]

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On Wed, 24 Sep 2014 16:39:55 +0100 Benjamin ESTRABAUD <be@mpstor.com> 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.

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...

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.

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.

NeilBrown

 
> 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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