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.
Regards,
John Gill
cisco
On 7/26/11 4:14 PM, Nick Hilliard wrote:
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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