[3509] in RedHat Linux List
Re: UPS reccomendations
daemon@ATHENA.MIT.EDU (Eric Smith)
Fri Nov 8 20:43:36 1996
Date: 9 Nov 1996 02:33:11 -0000
From: Eric Smith <eric@brouhaha.com>
To: Mac McClellan <list@digitalpla.net>
CC: redhat-list@redhat.com
In-reply-to: <1.5.4.32.19961108222423.00f0e0f8@mail.digitalpla.net> (message
from Mac McClellan on Fri, 08 Nov 1996 15:24:23 -0700)
Resent-From: redhat-list@redhat.com
Reply-To: redhat-list@redhat.com
Mac McClellan <list@digitalpla.net>
> I have three servers that I want to put on a UPS. Each server has a 300W
> power supply.
UPSes are generally rated for their maximum load in VA rather than Watts.
If you make the simplifying assumption that the power supply draws constant
current, then VA = 1.4 * W, so a 300 W load would be 420 VA.
However, the 300 W rating of your power supply is a maximum, and the actual
power consumption will vary depending on what is installed in the machine.
Another thing to consider is that while your UPS must be rated for at least
the VA of your load, there are reasons why you might want a higher rating:
1) A typical UPS is designed to power the rated load for about 5 to 10
minutes. A unit with a higher rated load will power the lesser load
for a longer period of time. In fact, lead-acid batteries are more
efficient if you discharge them at less than their peak current, so if
you were to compare two UPSes rated at 500 VA and 1000 VA that both could
power their rated load for 5 minutes, the 1000 VA UPS might be able to
power a 500 VA load for noticably more than 10 minutes.
2) Lead-acid batteries (even the "deep cycle" ones) degrade when they are
cycled. If you need to power the system for 5 minutes, you are better
off getting a UPS that is capable of powering the system longer, but
setting up the UPS or the software to turn it off sooner. That way the
battery doesn't get discharged as much, and will have a longer lifetime.
> What kind of UPS would be the best for my situation?
There are three main kinds of UPSes:
1) Standby Power Supplies: These have a relay to switch from the AC line
to the inverter (battery power) in an outage. They have a rated transfer
time in milliseconds, which should be as low as possible.
2) Online UPSes: These always run the load from the inverter, even when
AC power is available. The advantage is that there is no transfer time,
and that this provides voltage regulation to the load. These tend to
be very pricey.
3) Line Interactive UPSes: Despite reading a technical note purporting to
explain how Line Interactive UPSes work, I still don't really understand
it. These seem to combine the characteristics of the other two types.
APC claims that they are more reliable than other types.
Another thing to consider is the output waveform.
1) Cheap UPSes have a square wave output. Switching power supplies in
computers are perfectly happy with this. But other devices such as
monitors and printers, and any devices using power transformers, may
not be so forgiving. Transformers are substantially less efficient on
square wave input, so they use more power (draining the UPS battery
faster), and turn that extra power into heat.
I'm not sure whether anyone is still selling these.
2) Slightly more expensive units have modified square wave output. These
try to round the edges of the square wave a bit.
3) Quality units have true sine wave output.
The relevant measure of the output waveform is harmonic distortion; the
lower the distortion the better.
And the other main distinguishing feature is the computer interface.
1) No computer interface at all. Don't use this unless you are certain that
you only have to ride through brief power interruptions.
I'm not sure anyone sells these any more.
2) Simple signalling. These have one or more sets of relay contacts which
may be used to signal the computer. One set of contacts notifies the
computer that the AC power has failed. Typically another set indicates a
battery low condition. There may or may not be a way for the computer to
signal the UPS to shut it down. The init and powerd programs from the
SysVinit package (standard on Red Hat Linux) include support for simple
signalling.
3) Smart. These have an EIA-232 serial interface or a network interface for
SNMP. Usually they support dumb signalling also. Unfortunately some of
the major manufacturers try to keep their serial protocols secret to
force you to buy their expensive software (of which they have none for
Linux anyhow). Fortunately people have reverse engineered enough of
the protocol for the APC SmartUPS to make it work with Linux; there is
a package called apcd. I'm not sure whether there is an RPM for it.
> Should I get one big one for all three or three individual smaller ones?
There is some economy of scale to using one large unit instead of three
small ones. You can have one machine talk to the UPS, and notify the other
two via your network when they should shut down.
I personally prefer using a smaller UPS for each machine, with enough
excess VA rating that if one of my UPSes fails, I can plug a machine into
another machine's UPS while I get the broken UPS fixed.
> What about software to shutdown the servers when they run out of juice?
See the section on signalling above. I have my machines configured to
shut down and turn off the UPS after five minutes of AC outage. I bought
UPSes with sufficient rating and battery capacity to run my machines for more
than 30 minutes, so that won't cycle the batteries too much.
Cheers,
Eric
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