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

[Nick Hilliard]

On 26/07/2011 19:47, John Gill wrote:
> 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.

it's still quite common these days to have a 10g path from the san/nas to 
the network, but 1G access from the network to the client machines, 
modelled using a 10G distribution layer switch connected to both the 
san/nas and a bunch of ToR switches.  The ToR switches would have 10G 
uplinks, but 1G downlinks;  and unless these ToR switches have sufficient 
buffering capabilities, packet loss may occur on the 1G egress path.    So 
although cheaper, it can often prove to be false economy.  Also, 1GE is 
getting to be quite slow for disk access these days.  My 4.5yo laptop 
pushes 180 mb sequential reads, which is way more than 1Gbit/sec.

There's a rather good paper on C3560/C3750 egress qos here (thanks to Saku 
Ytti for the link):

https://supportforums.cisco.com/docs/DOC-8093

But as you point out, you can largely obviate the need for buffering by 
using the same speed throughout the network.  It's on this basis that the 
usual models of 10G ToR cut-thru switches make technical sense even though 
they invariably have tiny buffers, e.g. N5K or any of the other vendor 10G 
merchant silicon-based switches.

Nick