NFS and kernel cache

Chris Adams cmadams at hiwaay.net
Thu Dec 21 14:14:54 UTC 2006


Once upon a time, Les <hlhowell at pacbell.net> said:
> Ethernet only works well when the network is utilized about 50%.

I'm familiar with how Ethernet works, and this is not Ethernet that is a
problem.  Properly configured switched Ethernet can work without errors
at much more than 50% (I do that every day).  Gigabit Ethernet should
run much faster than 200Mbps.  When I ran a similar test with a Fast
Ethernet interface, I saw similar "stuttering" behavior, but it still
averaged around 70-80Mbps.

There is no router in my problem setup; this is two systems connected to
a switch (not a hub).  Neither is running anything else (one is booted
in rescue mode).  Both systems show full duplex 1000Mbps link with flow
control enabled (both have tg3 chips).  I was going to try jumbo frames
to see what difference that made, but one system doesn't support jumbo
frames.

The fact that Linux stops sending on the network sometimes and stops
reading the hard drive other times points directly to how the kernel is
caching writes to NFS (but I can't tell if it is the filesystem layer or
the network stack).
-- 
Chris Adams <cmadams at hiwaay.net>
Systems and Network Administrator - HiWAAY Internet Services
I don't speak for anybody but myself - that's enough trouble.




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