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