[CentOS] UPS question

Tue Sep 30 21:31:58 UTC 2014
Digimer <lists at alteeve.ca>

On 30/09/14 05:28 PM, Valeri Galtsev wrote:
> On Tue, September 30, 2014 2:13 pm, Digimer wrote:
>> On 30/09/14 03:07 PM, Valeri Galtsev wrote:
>>> On Tue, September 30, 2014 1:41 pm, Digimer wrote:
>>>> On 30/09/14 02:33 PM, John R Pierce wrote:
>>>>> so I have a bunch of servers at work that are on DUAL UPS's (1 per
>>>>> each
>>>>> of 2 power supplies).
>>> The only thing that stopped me from doing that was: you can only use
>>> each
>>> UPS up to a half of its spec'ed current drain.
>>> Thinking in line of: having 2 UPSes for rack, how do you distribute
>>> power
>>> from them to machines which luckily have 2 power supplies each (for
>>> redundancy). Having 1 PS wired to one UPS and 2nd PS to second UPS will
>>> save you if one of UPSes failed (and does not provide any output AC).
>>> But
>>> in this case you will need UPS as powerful (current drain wise) as to
>>> power the whole rack. So you are paying for reliability by using UPSes
>>> of
>>> double capacity. Did I not miss anything? I guess I almost did: if you
>>> have 4 UPSes for 2 racks you can wire them so that if only one of 4
>>> fails,
>>> you will have increase in draw of all UPSes only by 1/3...
>>> I decided _we_ are not that rich anyway...
>> In our case, that is what we do. We're an HA shop, first and foremost,
>> so *everything* has to be redundant. So yes, each UPS, on it's own, has
>> to be able to hold up all equipment for the minimum specified runtime.
>> That said, it's not really a "waste", because when there is a total
>> poewr out, we get twice the minimum hold-up time, which comes in very
>> handy at times.
>> For example, we had a client who runs Windows VMs on out system. There
>> was a major power out event that we knew was going to outlast the backup
>> power (they're a manufacturing facility, so if the machinery isn't up,
>> then the servers aren't doing much). After we determined that we had to
>> shut everything down, we found that someone had not turned of MS's
>> automatic updates. So one server decided that it was a great time to
>> install updates during a critical outage.
>> Thanks to having the extended runtime, we were able to shed some load
>> and hold up the host node and the server long enough for windows to
>> finish all of it's OS updates. Obviously, this should never have
>> happened in the first place, but it's an example of how extra runtime
>> can come in super helpful.
> Which asks for one more piece of equipment (hopefully you are not on a
> high floor...): diesel generator. That kicks in if the power doesn't
> return after some short outage. (I was almost sure you mention it
> somewhere closer to the end...)
> Valeri

Some of our clients have that, but not all of them. Even the ones that 
do, though, can have trouble. A different recent client had a major 
incident in their power distribution room (fire and/or explosion). They 
had enough fuel for 6~8 hours of operation. They couldn't get more fuel 
in time and ended up having to shut down their production facility until 
they could arrange a more long-term solution.

You can go to great lengths to protect yourself, but there will always 
be limits. It's a question of where your paranoia and budgets meet.

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