Sunday, 27 November 2011

Explosion in power needs

In the later years there has been an explosion in the power requirement per rack.  Not long ago you got 2*16amp sockets, for a and b side, and that was it. And it was like that for 10 years.  Then came the higher density of blades where 16 servers could now fit in a space before populated by 10 or sometimes jut 5.  On top of that each server would have more cores and each chassie would have to have psu’s to cater for it’s top spec  Pretty fast you are requiring more like 4*32amp per 10u and fuses where tripping all over the place.
Yes you can power manage by limiting the power each server and chassie can use, but then you can never run at your top capacity, so why did you buy it.  You will also have startup issues if you have total power failures.

For security against the frequent failures or just scheduled maintenance of the normal power grid most companies with in-house servers has some form of a ups system.  Here the problem is they seldom last for more than 10 or 20 minutes if you are lucky.  They will be based on batteries and batteries are not a good way of storing any significant amount of power when it comes to appliances that use large amounts at 220v. 
And what can you do in let’s say 15 minutes.  It’s hardly enough time for an admin to shut down the most essential databases.  (Oracle do not enjoy a sudden and complete loss of power).  Most will use best part of that time to trigger the alert.  Here an environment monitor like Avtech is worth its weight in gold for fast sms notification.
Most companies above a certain size will backup their ups with a generator.  And I do say “a” because very few beside dedicated data centres that offer services to third parties, has more than 1.  What they forget is a generator is more like a car.  How sure are you that your car will start first time after standing idle for a few weeks.  Regular testing is required but most generators stand around for many years, so now we are talking about a 20 year old car.  Yes it doesn't have much mileage, but that is not always a good thing.  Diesels like to be run.
If you try to solve this by a second generator you are in for a very complicated and vulnerable fail over system, to ensure that every part is redundant.  And somewhere in the middle there will be a some sort of a vulnerable failover switch.  Remember also you don’t want to make it so complicated that it induces more risk than what you where guarding against.

You could try to get a second grid supply but in most places you will find that an actual physical separation on the supply side is nearly impossible. Competition just hasn’t got that far.  You will also run into the same problem as for a second generator, how to feed power from 2 sources.

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