I was going to add some more details to Chuck's post on how a blade server powers on, but I got sidetracked by a brilliant post from Mike Manos of Digital Realty on the real basics of what is going with power in your datacenter.

What Mike is explaining, far better than I could, is how power gets used up and reserved in your datacenter by breaker sizes, redundancy and natural tendency of the facility management to be conservative when allocating power to servers, and as he says they have good reason to be.  If they plug in a device that causes a breaker to trip taking down multiple servers - it's their butts that are on the line.

He raised a good question about why the faceplate label, the label on power supply that indicates the max power input, is so high that most facilities managers are comfortable de-rating it by 20% - 30%.  Well the reason is explained in part by my post on how configuration affects power consumption; the power supply is designed to deal with maximum configured load.  The range from a minimum configured load for a 2 socket server e.g. 1 Low Power CPU, 1 or 2 DIMMs, 1 x SSD drive and no PCI cards, to a maximum configured load e.g. 2 x 120W or 130W CPUs, 12 or 18 DIMMs, 8 x 15K RPM Drives, 3 x PCI Cards including a 200W graphics card, is huge and that’s just one server.  The example I use in the Configuration Matters post shows a difference of over 1kW across an enclosure.  Talk to any power supply designer and you'll find out that they are just as conservative as any facility manager (and unappreciated) and for pretty much the same reasons.  Who gets blamed when you run a high power program like Prime95 or Linpack and the server shuts down because the power supply couldn’t deliver enough juice.

That’s why HP came up with the common slot power supply design for rack mount servers. It allows you to size the power supply for the actual configuration you will be using rather than just stuffing a 1200W power supply in every server.

 This has two great consequences:

1. It reduces the amount of trapped or stranded power by reducing the amount the power that the facility manager has to allocate to a given server.
2. It increases your power supply efficiency, reducing energy wasted.  All power supplies have an efficiency curve that for servers at low outputs has a low efficiency and gets to peak efficiency at about 35% - 50% 65% load (Got corrected by one of the engineering team on this.  Must remember in future to check my numbers). Remember most servers have redundant power supplies and in the HP case they load share so the PSU can only ever exceed 50% load in the event of a redundancy failure.

This does add complexity to your buying decision, now you have to pick the power supply you need based on your configuration.  That's why we created the HP Power Advisor to help with that decision.  Of course you can still just use a 750W or 1200W PSU for every server if you want to, but you won't be running as efficiently as you could.

One area though where I must respectfully disagree with Mike is in his comments on Power Capping. I agree that is a technology that has huge potential in the datacenter to allow your facilities team to recover that trapped capacity, but I disagree that it is not ready for prime-time.

HP delivered our first version of power capping in 2007. This was relatively slow acting and was really only good for controlling the average power consumption of a server. This was great if you had a cooling issue on your datacenter and wanted to control the heat output of your servers as heat is largely related to average power of the server, but you couldn’t use it to protect circuit breakers.

In November 2008 HP introduced Dynamic Power Capping with circuit breaker protection.  This is a hardware based solution that can respond to changes in power consumption in less than 500ms and because it’s a hardware solution it’s operating system and application independent.  This is supported on all G6 servers, most blade servers and selected G5 rack-mount servers.  When run on an HP Blade Enclosure you gain additional capabilities; the Onboard Administrator can manage the blade server caps to optimize the performance of the enclosure. It will change the blade level power caps so that busier blades get more power and less busy blades will get less power while maintaining the enclosure level power cap so you can protect your breakers.

For a demonstration of this on the rack mount servers showing how we deliver circuit breaker protection see this video with “Professor” Alan Goodrum and for more information Dynamic Power Capping go to http://www.hp.com/go/powercapping