Wednesday, 6 June 2007


Quite a while back, I posted on the Green Datacentre. Actually, the post was over a year ago and datacentre power and cooling issues have become a really hot (sic) issue.

However, I have to castigate myself severely for not Reading the Flaming Manual. In particular the power demands of the Cisco 9513 switch. Really the problem comes from working in an environment where power/cooling isn't wasn't an issue - just whack it in and get the electricians to provision whatever power you need. No need to actually validate the power requirements - just overprovision and everything will be fine. But recently, I wanted to power a 9513 up outside the datacentre to do pre-installation checks. The question was, could I power it up from a normal 13A socket? With 6000W of power required, surely not! Well, this is where RTFM comes in.

Thumbing through the 95xx Installation Guide I found that the chassis and fan trays take 318 watts, supervisor cards take 126 watts each and the biggest module, 48 port line-cards, 195 watts each. Therefore a fully populated 9513 would require 2715 watts. Based on UK power supplies of 240V, that's just over 11 amps. So, technically (although I wouldn't advise it and probably wouldn't try it) a 13A power supply would suffice.

So what's with the 6000W power supply? In fact, the power supplies in the 9513 are 6000W *capable* but don't have to demand or supply that level of power. Each takes two power feeds (from separate PDUs hopefully) and if only one feed is available, the power supply can only provide 2900W. If you think about it, what that means is that overall, 2 power supplies can provide ample power to run the switch without being under stress (and therefore provide more consistent, smooth power). Should a PDU fail, then each PSU can supply 2900W, again easily enough to run the whole switch. If *at the same time* one PSU should also fail, the remaining power supply can still run the whole switch. It may not be a desirable situation but it is possible. In terms of what power/cooling requirement should be catered for, it is actually a maximum of 2715 watts.

It just shows what reading the manual can do for you.

1 comment:

Stephen said...

In my last job, everything in our data centre was on UPS. We had two of them rated at some outstanding AMP's. Problem was we used three phases and we tried very hard to balance the phases.

When we put in the MDS 9509's, I tried to figure out what the power supplies actually pulled. Did each power supply pull all the power or did they load balance. The ones we got were 2400 watt. So, I argued with the power people for weeks on what the damn things were going to pull. To be honest, I still don't know how the power is used on the MDS. I calculated the amperage draw to cater for losing one of the UPS.

Even after RTFM, I am still wondering if it load balances the power. I personally doubt they do.

The same thing goes for the HDS USP's. I doubt they load balance the power and that was another drama I had with the power people.

Actually the same thing went with our large Sun Fires. They decided which power supply they wanted to run on. If I tripped them over to UPS B, sooner or later they would end back on UPS A. So load balancing a large datacenter for power and UPS is a nightmare.