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
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