Over-subscription question.

[stephen2615]

Greetings,

I have a very good grasp of over-subscription but I would like to know what the following means. It was taken from the following URL:

http://www.cisco.com/en/US/products/ps5990/products_data_sheet0900aecd80446058.html

<snippet>

The Cisco 48-Port 4-Gbps Fibre Channel Switching Module delivers 96 Gbps of total bandwidth

</snippet>

Actually all the second generation cards seem to offer 96 Gbps of bandwidth. Could someone please explain that number and why only half of its bandwidth is available for the actual ports. Simple maths would suggest that 96/48 would give 2 Gbps absolutely worst case speed.. right?

Cheers

Stephen

[dmcloon]

Hi Stephen,

It is oversubscribed if all 48 ports are running at 4Gbps and sending at wire rate, which is highly unlikely in real world. At 2Gbps each port can sustain line rate.

Cheers

Dallas

[stephen2615]

Hi Dallas,

Another snippet..

The 48-port linecard offers line-rate throughput at 1 Gbps but is 2:1 and 4:1 over-subscribed respectively for 2 Gbps and 4 Gbps, if all ports in the same port group attempt to push line-rate traffic simultaneously.

I read that as each port is able to sustain only 1 Gpbs.

With good design, thats not really going to be a problem with our current systems. I just wonder about the different throughputs that appear on the pages.

Does each Gen 2 card actually have 96 or 48 Gbps bandwidth? It looks like 48 to me.

I have been watching our Windows servers and they don't do much over 30 MBs when bashing themselves around. Unlike the SunFire systems at a previous job that pushed upwards of 190 MBs when doing in system HDS array backups..

Almost time for a VB..

Cheers

Stephen

[dmcloon]

Yes your right. It is oversubscribed 2:1 at 2Gbps. There is 48Gbps for each gen2 module. All mods have 4 port groups and each port group 12.8Gbps uplink to cross bar.

- 48 port card has 12 ports per port Group

- 24 port card has 6 ports per Port Group

- 12 port card has 3 ports per Port Group

- 4 port 10Gbpc card has 1 port per Port Group

[skurganov]

96 Gbps of total bandwidth mean full-duplex bandwidth - 48 Gbps in each direction. A bit marketing number..

[stephen2615]

I found a very interesting article on the brocade web site.

http://www.brocade.com/san/pdf/whitepapers/Achieving_Ent_SAN_Perf_48000_Dir_WP_01.pdf

It appears that Brocade have finally admitted to over-subscription. It makes very interesting reading if you look between the lines. I wish I was a marketing guru.

Stephen