[CentOS] UPS question

Tue Sep 30 21:28:06 UTC 2014
Valeri Galtsev <galtsev at kicp.uchicago.edu>

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 Galtsev
Sr System Administrator
Department of Astronomy and Astrophysics
Kavli Institute for Cosmological Physics
University of Chicago
Phone: 773-702-4247