Friday, June 27, 2008

Data Center - Take 2

Shortly after my last post - actually it was the next morning in the shower, which is where I do my best thinking - I was hit by the thought, "doesn't CX4 reach 15 meters instead of 10?". So when I got to work that morning, I looked it up and, sure enough, 10GBASE-CX4 has a 15 meter (49 foot) reach. And that, my friends, makes all the difference !

15 meter reach makes it possible to use CX4 as the uplink between all the TOR switches in a 26 cabinet row and a distribution switch located in the middle of said row. This reduces the cost of each TOR switch 10GbE uplink by several thousand dollars.

The result is that, for a server rack with 48 GbE ports, it's substantially less expensive to deploy a TOR switch with 2 10GbE (CX) uplinks to a distribution switch than to deploy a two 24-port patch panels with preterminated Cat6e cables running back to the distribution switch. For server rack with 24 GbE ports, it's a wash in terms of cost - the TOR switch option being a few percent more expensive. This also means that the cost of 10G server connection is significantly lower than I originally calculated.

The only remaining issue is that, in the new data center, the plan was to distribute power down the middle aisle (13 racks on each side of the aisle) and out to each rack, but to distribute the fiber from the outsides of the rows in. One thing that makes the TOR model less expensive is that you only need 1 distribution switch per 26 racks (13 + 13) whereas with the patch panel model you'd need multiple distribution switches on each side of the aisle (2 or 3 switches per 1/2 row or 13 racks). But having only 1 distribution switch per row means that there would be CX4 cables and fiber crossing over the power cables running down the middle aisle. We have 36" raised floors though, so hopefully there's plenty of vertical space for separating the power cables and network cables.

The other consideration is that it appears to me vendors will be converging on SFP+ as a standard 10G pluggable form-factor - going away from XENPACK, XFP and X2. If this happens, SFP+ Direct Attach will become the prevalent 10G copper technology and that I believe does only have 10 meter reach. That would lead us back to placing a distribution switch on each side of the aisle (1 per 13 racks instead of 1 per 26 racks) - which will raise the overall cost slightly.

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