[CentOS] UPS question

Tue Sep 30 19:13:28 UTC 2014
Digimer <lists at alteeve.ca>

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.

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