NFS auto-reconnect tuning.

Post by Benjamin ESTRABAUD
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).
[ 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

Post by Benjamin ESTRABAUD
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.
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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Benjamin ESTRABAUD

2014-09-25 09:46:09 UTC

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

Post by NeilBrown
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.

Post by NeilBrown
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!

Post by NeilBrown
__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.

Post by NeilBrown
NeilBrown

Regards,

Ben - MPSTOR.

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NeilBrown

2014-09-28 23:28:36 UTC

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).
[ 1663.940406] nfs: server 10.0.2.17 not responding, still trying
[ 2712.480325] nfs: server 10.0.2.17 OK

This is great! I had no idea, I will definitely roll NFSv4 and try that.
Thanks again for your help!

Actually ... it turns out that NFSv4 shouldn't close the connection early
like that. It happens due to a bug which is now being fixed :-)

Probably the real problem is that the TCP KEEPALIVE feature isn't working
properly. NFS configures it so that keep-alives are sent at the 'timeout'
time and the connection should close if a reply is not seen fairly soon.

However TCP does not send keepalives when the are packets in the queue
waiting to go out (which is appropriate) and also doesn't check for timeouts
problem when the queue is full.

I'll post to net-dev asking if I've understood this correctly and will take
the liberty of cc:ing you.

NeilBrown

Benjamin ESTRABAUD

2014-09-29 10:06:26 UTC

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).
[ 1663.940406] nfs: server 10.0.2.17 not responding, still trying
[ 2712.480325] nfs: server 10.0.2.17 OK

This is great! I had no idea, I will definitely roll NFSv4 and try that.
Thanks again for your help!

Actually ... it turns out that NFSv4 shouldn't close the connection early
like that. It happens due to a bug which is now being fixed :-)

Well, maybe I could "patch" NFSv4 here for my purpose or use the patch
you provided before for NFSv3, although I admit it would be easier to
use a stock kernel if possible.

Post by NeilBrown
Probably the real problem is that the TCP KEEPALIVE feature isn't working
properly. NFS configures it so that keep-alives are sent at the 'timeout'
time and the connection should close if a reply is not seen fairly soon.

I wouldn't mind using TCP Keepalives but I am worried that I'd have to
change a TCP wide setting, which other applications might rely on (I
read that the TCP keepalive time for instance should be no less than 2
hours). Could NFS just have a "custom" TCP keepalive and leave the
global, default setting untouched?

Post by NeilBrown
However TCP does not send keepalives when the are packets in the queue
waiting to go out (which is appropriate) and also doesn't check for timeouts
problem when the queue is full.

So if I understand correctly, the keepalives are sent when the
connection is completely idle, but if the connection break happened
during a transfer (queue not empty) then NFS would never find out as it
wouldn't send anymore keepalives?

Post by NeilBrown
I'll post to net-dev asking if I've understood this correctly and will take
the liberty of cc:ing you.

Thank you very much for this, this will help.

Post by NeilBrown
NeilBrown

Ben - MPSTOR

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NeilBrown

2014-09-29 21:34:15 UTC

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).
[ 1663.940406] nfs: server 10.0.2.17 not responding, still trying
[ 2712.480325] nfs: server 10.0.2.17 OK

This is great! I had no idea, I will definitely roll NFSv4 and try that.
Thanks again for your help!

Actually ... it turns out that NFSv4 shouldn't close the connection early
like that. It happens due to a bug which is now being fixed :-)

Well, maybe I could "patch" NFSv4 here for my purpose or use the patch
you provided before for NFSv3, although I admit it would be easier to
use a stock kernel if possible.

You could. Certainly safer to stick with stock kernel if possible (and we
appreciated the broader testing coverage!).

That is exactly what NFS does - it sets the keep-alive settings just for the
TCP connection that NFS uses.
The problem is that TCP keep-alives don't quite work as required.