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

[Nick Hilliard]

On 27/07/2011 05:00, John Gill wrote:
> This paper is indeed a great read, it goes over some really good points of
> the buffering and queuing that are commonly misunderstood.
>
> All good points in this thread, I will make one small comment about the N5k
> here if one wanted to compare buffer sizes on paper: The N5k uses ingress
> buffering with virtual output queues. So when you oversubscribe a single
> egress interface, buffers available for use are proportional to the number
> sending to that interface. It essentially acts like a shared buffer.

thing is, you can't really do a paper comparison of the n5k and 3560/3750 
buffers - one model is store-n-forward, while the other is a cut-thru. 
This means that even though the n5k has relatively modest buffers by 
comparison, they go much further during normal operation because they're 
not routinely used unless there is port congestion.  Unless of course, you 
mix-n-match 1G and 10G on the same chassis, which causes the n5k to 
implement per port store-n-forward on the 1G <-> 10G paths.  However, this 
is usually a very poor idea (unless you know what you're doing).

Interestingly, the Miercom-supervised Nexus5k vs Arista 7124 lab test of 
April 2010 uses ingress buffering + VOQs on the N5K to concoct some of 
their more extraordinary (but fully repeatable) results.  I love that 
paper: the comparison methodology between the two switches is completely 
hilarious.

Nick