[c-nsp] Flow Control and 10GE interfaces

[Ross Vandegrift]

On Sun, Nov 22, 2009 at 08:28:24PM -0000, Matthew Melbourne wrote:
> What is the general recommendation regarding enabling flow control on
> Ethernet interfaces. Is it a legacy issue when devices had smaller buffers,
> or is it still required for specific applications? We are having issues with
> an Enterprise NAS solution where servers using it for storage are claiming
> to be losing connectivity. The NAS is connected to the switch fabric (a pair
> of Catalyst 6509s) by two 2*10GE port-channels (10GBase-SR optics); receive
> flow control is enabled on the switch side "flowcontrol receive on", but no
> input or output pause frames are being received/sent according to the member
> interface statistics. 
> 
> The vendor is now suggesting that flowcontrol needs to be enabled end-to-end
> - e.g. on aggregation switches downstream from the Catalyst 6509s towards
> the servers and on the hosts. However, the utilisation on the NAS
> port-channels is only ~400Mbps. Does enabling flowcontrol make sense here?

Storage vendors seem to blame a plethora of issues on disabled
Ethernet Flow Control.  Every discussion that I've ever had with any
of them, every document that I've ever read, totally fails to
understand what ethernet flow control does and how it works.  No one
is even aware of the head of line blocking problem.  Remember - when
you pause your NAS, you pause it for EVERYONE.  Maybe I've talked to
the wrong folks, but no one seems to understand this.  It's almost
like EMC thinks they designed their NAS for a single client...

The answer is very simple: if someone thinks that ethernet flow
control is the answer, the burden of proof is on them to answer
difficult questions about what the actual problem is, what flow
control is going to solve, and why they think that it won't cause more
problems than its worth.  At best it does nothing, realistically it
interferes with TCP flow control, and at worst it pauses your storage
and breaks every client.

-- 
Ross Vandegrift
ross at kallisti.us

"If the fight gets hot, the songs get hotter.  If the going gets tough,
the songs get tougher."
	--Woody Guthrie
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