[c-nsp] iSCSI, port buffers, and small switches

[Peter Rathlev]

On Tue, 2011-07-26 at 14:47 -0400, John Gill wrote:
> The customers I have come across with buffering problems on 3750 were
> either not effectively configuring QoS - sometimes the best config is
> no QoS actually.  The buffers get divided up when you enable "mls qos"
> and if you don't use all the queues as they are set up, they will not
> be optimal, especially for the default class.

Hmm... this specific part has me a little confused. As the archives
mention in detail the "apparently too small buffers" on the 3560/3750
family of switches is not an uncommon problem. We have ourselves worked
hard to come up with a solution, since replacing several hundreds of
switches would be prohibitively expensive.

The most successful of the solutions has been to actually configure QoS
and re-partition the buffers. This makes me think that a switch in this
family with "no mls qos" does not actually use the available buffers
fully.

I've talked with several SEs and some marketing boss (following up on a
survey) about this, and none have been able to give any answers without
having me sign NDAs. I'd rather not do that, since we have an acceptable
solution until these switches are naturally replaced.

We haven't fully tested the newest software (12.2(55)SE and later where
the switch does a lot of ASIC reprogramming on startup) to see if the
problem persists.

> The big scenarios you need buffering are speed difference (regardless
> of average rate!) or many-to-one host at the same time, otherwise you
> don't need significant buffers at all.

This is an important point IMO. If you don't need any kind of traffic
differentiation and have ample bandwidth there should be no problems
with small buffers.

-- 
Peter