I explain the reservation in the data center on beer

We write a lot about serious things from the world of data centers: about the requirements for data centers, certification, and the design of engineering systems. But in our busy times I want posts for both good and mood. Today I propose to take a fresh look at such a complicated thing as certification of a data center according to the Tier standard from the Uptime Institute. I will explain the reservation requirements for beer.



The Uptime Institute standard evaluates the data center and assigns it one of 4 levels: Tier I, Tier II, Tier III and Tier IV. The higher the level, the better the data center should think out insurance against various risks, for example: if the equipment fails or the power goes out.
Reservation - and there is such insurance just in case. For different levels of Tier, you need a different level of redundancy: N + 1, N + 2, 2N or 2 (N + 1). N is the minimum amount of equipment without which the data center will not be able to work. I note that for backups, not only the stock in terms of the quantity of equipment is important, but also spare distribution routes: power cables, routes, pipelines and so on. We will also talk about this.  

So, let's imagine that we are not in a data center, but in an ordinary apartment. In the evening we will need 2 bottles of beer. Two bottles of beer is N. 


 
What reservation looks like from this point of view according to Tier standards:

Tier 0 : We know that we need 2 bottles, but we deliberately deceive ourselves and buy one.


There is nothing to comment on: the fools themselves. 

Tier i . We buy exactly 2 bottles of beer. Everything seems to be fine, but if one bottle breaks or becomes expired, then we will not have enough beer.



In the data center, this would mean that there is no backup equipment. Any breakdown - and the work will stop. 

Tier II . We are taught bitter beer experience and buy 3 bottles of beer, with a margin. This is already a form of N + 1 redundancy.

Well done, we reserved beer, but we carry all 3 bottles in one package. We did not reserve distribution routes: if the package breaks, we will lose all beer. 



In the data center, it looks like this: we already have a backup element, for example, a spare air conditioner. But if we power it from one source, we will not be able to insure this air conditioner from power outages. 

Tier III . We need to reserve both the bottles themselves and the distribution routes. How to do it?

The first option: we buy two packages, insert one into the other and carry it! A good, proven option, and most importantly - reliable. Beer is less likely to break. For example, if you knock it out of hand.  

In the data center, this would mean that we have a spare air conditioner and 2 power sources are connected to it.      

The second option: we buy 4 bottles and 2 packages, in each we put 2 beers. This is a 2N reservation. We take one packet in one hand, the other in the other. The option is extremely reliable, here we even swing at Tier IV, but, unfortunately, expensive. 

Everything is duplicated in such a data center: there are 2 main power supplies from different transformers, each of them is duplicated by a backup power supply from diesel generator sets, air conditioners are 2 times more than necessary, and each has 2 power supplies from different beams, and so Further.  

We can make it a little cheaper: buy only 3 bottles of beer, but 2 bags, and put the bags together. One will tear - it's okay.  



Tier iv. Here we have a new scary word, practically a check - whether there are a lot of beer drunk. Try it out the first time: compartmentalization of distribution paths!

For the data center, this would mean that on all sections of the route, pipelines, power cables should go in different ways, so different that at least a fire, at least a flood, at least a terrible excavator.

So it is with us with our valuable cargo: even a fire, even an angry dog, even gopnik around the corner. 

Here I need help from a friend. Each of you should go to different stores, buy 4 bottles, distribute them two pieces in different packages, take in different hands and go in different ways. You can only meet in the apartment. No matter what happens, you still get your required N.



How will it be in the data center: two transformers, 2 diesel generators, 2 UPSs - and all this in different rooms, and the power cables go in different ways, protected like a president’s motorcade. 

Complicated? And it can be even more complicated: the current reservation level is 3/2  
(there is also 6/5, but it needs to be explained on stronger drinks). 

We call a third friend, each with a bottle in each hand, another one in the backpack behind him. Although packages do not need to be bought, that’s good. Each goes on different roads from different stores, and God forbid the paths intersect. What is it for? Suppose one person broke all three bottles, and he returned home sad. Two friends coped with the task, and you three have 6 left - exactly as much as you need!



Thus, we save on the total quantity, but complicate the process of power distribution between consumers. After all, if the three of us gathered, how could we stop at two and distribute them correctly for everyone ... But this is a completely different story.

Z.Y. In one place, I exaggerated the requirements of the standard a little. The first attentive reader of Habr, who will notice this, is a beer reserve as a gift. Write the options in the comments.