[ale] network question

[James Taylor]

I've been working with optimization for iSCSI lately, and jumbo frames are big factor for it.
Everything that I've seen or heard indicates that if any device on switch doesn't support jumbo frames, they get disabled.

Some of the switches support setting up jumbo frames in isolated vlans or even ports on the same switch, but any device that doesn't support jumbo frames will disable it for the rest of the devices on the same network.

So far the only devices I've worked with have been current generation Extreme switches
When we enabled jumbo frames for the linux boxes, the storage throughput increased about 30%.

-jt


James Taylor
The East Cobb Group, Inc.
678-697-9420
james.taylor at eastcobbgroup.com
http://www.eastcobbgroup.com












>>> "James P. Kinney III" <jkinney at localnetsolutions.com> 08/26/07 10:43 PM >>> 
If I have a server with a gig NIC connected to a switch with a gig NIC I
can turn on jumbo frames on both and have each tcp transaction be more
payload and less overhead.

OK

So if there is another switch one more hop away from my server and it
also has jumbo frames on the traffic continues is a more efficient more
from the server down to the second switch.

But jumbo frames are not supported by 100Mb NICs ( at least none I've
seen) so jumbo frames won't work if the final data recipient doesn't
support them. I'm guessing that the result of this will be an extra
burden on each transmission setup to establish what the little one can
handle. Not a good thing as I'm trying to squeeze the maximum bandwidth
and minimum latency out of my network (tubes for the unknowing :).

So here's the final scenario: Same server and switches as before but add
the following twist - the server has 4 gig NICs bonded to do 802.3ad
(link aggregation) so they can do basically ethernet striping across 4
ports so maximized bandwidth between AoE hard drive stack (w/ a 10G
connection). But those same 4 wire bonds also talk down the switch
connections to the downstream little devices. 

So the Linux question of the day is: is it possible to have a variable
frame size based on destination or is the downsizing process of the
frame rate so insignificant it won't matter? Or will the last switch in
the chain handle the frame size conversion (SMC SMC8748L2
10/100/1000Mbps Managed Layer 2 Switch - still digging through the 480
page users manual).

-- 
James P. Kinney III          
CEO & Director of Engineering 
Local Net Solutions,LLC        
770-493-8244                    
http://www.localnetsolutions.com

GPG ID: 829C6CA7 James P. Kinney III (M.S. Physics)
<jkinney at localnetsolutions.com>
Fingerprint = 3C9E 6366 54FC A3FE BA4D 0659 6190 ADC3 829C 6CA7