Mike Manos responded to my post about power capping being ready for prime time with a very well thought out and argued post that really looks at this from a datacenter manager's perspective, rather than just my technology focused perspective.

I'm going to try and summarize some of the key issues that he brings up and try to respond as best I can.

Critical Mass

This one spans a number points that Mike brings up,  but I think the key thing here is that you must have a critical mass of devices in the datacenter that support power capping otherwise there is no compelling value.  I don't believe it is necessary, however, to have 100% of devices in the datacenter that support power capping.  There are two reasons why:

1.      In most Enterprise datacenters the vast majority of the power for the IT load is going to the servers.  I've seen numbers around 66% servers, 22% storage and 12% networking.  This is a limited sample so if you have other numbers let me know I would be interested.

2.      Most of the power variation comes from the server load. A server at full load can use 2x - 3x the power of a server at idle.  Network switch load variation is minimal based on some quick Web research (see Extreme Networks power consumption test [PDF]or Miercom power consumption testing). Storage power consumption variation also seems to fairly light at no more than 30% more than idle. See Power Provisioning for a Warehouse-sized Computer [PDF] by Google 

So if our Datacenter manager, Howard, can power cap the servers then he's got control of the largest and most variable chunk of IT power.  Would he like to have control of everything, absolutely yes, but being able to control the servers is more than half of the problem.

Been there done that, got the T-Shirt

The other thing that we get told by the many Howards that are out there is that they're stuck.  They've been round and round the loop Mike describes and they've hit the wall.  They don't dare decrease the budgeted power per server any more as they have to allow for the fact the servers could spike up in load, and if that blows a breaker taking down a rack then all hell is going to break lose.  With a server power cap in place Howard can safely drop the budgeted power per server and fit more into his existing datacenter.  Will this cost him, sure, both time to install and configure and money for the licenses to enable the feature. But I guarantee you that when you compare this to cost of new datacenter facilities or leasing space in another DC this will be trivial.

The heterogeneous datacenter

I agree most datacenters are in fact heterogeneous at the server level either; they will have a mix of server generations and manufacturers.  This again comes down to critical mass, so what we did was enable this feature on the two of the best selling servers of the previous generation, DL360 G5 and DL380 G5 and pretty much all of the BladeSystem blades to help create that critical mass of servers that are already out there, then add on with the new G6 servers.  We would of course love for everyone with other manufacturer's product to upgrade immediately to HP G6 ProLiant Servers and Blades, but it's probably not going to happen.  This will delay the point at which power capping can be enabled and for those customers that use other vendors systems they may not be able to enable power capping until those vendors support it.

Power Cap Management

There's a bunch of issues around power cap management that definitely do need to get sorted out.  The HP products do come from an IT perspective and they are not the same tools that facilities managers typically use.  Clearly there needs to be some kind convergence between these two toolsets even if it's just the ability to transfer data between them.  Wouldn't it be great if something like the Systems Insight Manager/Insight Power Manager combination that collects power and server data could feed into something like say Aperture (http://www.aperture.com/) then you'd have the same information in both sets of tools.

The other question that we have had from customers is who owns and therefore can change the power cap on the server, the facility/datacenter team or IT Server Admin team.  This is more of a political question than anything else, and I don't have a simple answer, but if you are really using power caps to their full potential changing the power cap on a server is something that both teams will need to be involved in.

I would like to know what are the other barriers you see to implementing power capping - let me know in the comments and be assured that your feedback is going into the development teams.

SNMP Access.

Just to make Mike happy I thought I'd let you know that we do have SNMP access to the enclosure power consumption.

If you collect all six SNMP MIB power supply current output power values and add them together, you will have calculated the Enclosure Present Power.

In the CPQRACK.MIB file, which you can get from here http://h20000.www2.hp.com/bizsupport/TechSupport/SoftwareDescription.jsp?swItem=MTX-a7f532d82b3847188d6a7fc60b&lang=en&cc=us&mode=3&

There are some values

cpqRackPowerSupplyCurPwrOutput which is MIB item: enterprises.232.22.2.5.1.1.1.10.1 through enterprises.232.22.2.5.1.1.1.10.6 gives you the Input Power of each Power Supply, I know the MIB name says output but it's actually input - sum these together then you have the Enclosure Input Power.

Power supplies placed in standby for Dynamic Power Savings will be reporting 0 watts.

And for enclosure ambient temp - read:

CPQRACKCOMMONENCLOSURETEMPCURRENT

Tony