[plug] NFS Large File writing causing "freezes"

[Tim]

On 26 September 2010 20:27, Patrick Coleman <blinken at gmail.com> wrote:
> On Sun, Sep 26, 2010 at 5:50 PM, Tim <weirdit at gmail.com> wrote:
>>           RX packets:134427956 errors:31444 dropped:31444 overruns:0 frame:0
>
> This is bad - something in the path to this host on the network is
> corrupting network packets. Check duplex settings, cables, NICs,
> switches.

According to the switch, everything is connected at full duplex 1Gbit.
All the cables are proper Cat5 and fairly short. (Not the silly cables
they give you with routers now days that only have 4 wires instead of
8).

>
> I'm guessing the freezes you're seeing are NFS blocking system IO as
> TCP tries to retransmit the data or ack packets that the network ate.
> Mounting the NFS share with the 'soft' option on the client might
> alleviate this, as it will let you interrupt outstanding nfs
> operations (though it won't solve the underlying issue - your network
> is dropping packets).

It's already mounted with soft, but I've not needed to kill any
process yet as it eventually finishes successfully.

I've tried a NFS mount between the 2 machines (so not to the NAS) and
can do a transfer at between 20 and 30MB/s without any RX errors on
the "server". It also doesn't appear to exhibit any freezes, or if
they are freezes then they are brief for only a second instead of upto
30 seconds.

I'm thinking that the client is sending data too fast for the
receiving NAS (which is only got 256Mb ram, 800Mhz arm processor), and
hence the dropped packets. Other than a really small wsize, any other
ideas for slowing down the transmit rate to something the NAS can
handle? From watching the traffic graphs, it seems to ramp up the
speed until it looses packets (>12MB/s) and then slows it down while
retransmitting packets (to about 2MB/s) and then once the queues are
cleared, it trys to ramp it back up again and the cycle continues.
Setting a small wsize (1024) means the transfer goes at a steady
2-3MB/s without long freezes or many RX errors.

I think I am realizing that if you want a decent NAS, building it
yourself might not be such a bad idea. Having plenty of RAM and CPU
would make things a lot better. (Although having a nas like the QNAP
means it's quite, cool, and low power).


Any ideas welcomed.

Thanks

Tim
-- 
Timothy White - Somewhere in Australia