If all the servers have redundant PSUs (), you can plug just enough PSUs into the UPs and rest into the main powergrid.
If you have 10 servers, with dual PSUs each (which makes 20 PSUs to plug into the power), you can simply plug only one of them into the UPS (so you would only take up 10 sockets in the UPS's PDB) - in case of power failure, the UPS will hold the servers up.
However, this does not cancel the previous comments about a UPS not being the ultimate answer to power failures.
Currently when one of the two UPS dies, the doshutdown event is triggered, and executes the default script via apccontrol. The doshutdown script ignores the second UPS, as they are not event-connected, and proceed normally with the shutdown.
In order to have the doshutdown events somewhat connected, the two instances of apcupsd need a specifically customized configuration file. The difference will reside in the directory from which the events scripts have to be executed.
Main properties of first ups, in /etc/apcupsd/apcupsd.ups0.conf
SCRIPTDIR /etc/apcupsd/ups0
UPSNAME ups0
DEVICE /dev/ups0
PWRFAILDIR /etc/apcupsd/ups0
NOLOGINDIR /etc/apcupsd/ups0
NISPORT 3551
EVENTSFILE /var/log/apcupsd.0.events
And for the ups1, in /etc/apcupsd/apcupsd.ups1.conf
SCRIPTDIR /etc/apcupsd/ups1
UPSNAME ups1
DEVICE /dev/ups1
PWRFAILDIR /etc/apcupsd/ups1
NOLOGINDIR /etc/apcupsd/ups1
NISPORT 3552
EVENTSFILE /var/log/apcupsd.1.events
Each scriptdir should get a copy of the default scripts.
We want to customize the doshutdown script, which will not directly shutdown the machine, but has to check if the other UPS is still on, or is in shutdown mode.
At the top of the doshutdown script, we could add something like
for ups0
if [ ! -f /tmp/ups1.is.down ]
then
touch /tmp/ups0.is.down
exit 99
fi
for ups1
if [ ! -f /tmp/ups0.is.down ]
then
touch /tmp/ups1.is.down
exit 99
fi
the status 99 has a special meaning, that tells apccontrol to stop the action in progress. The five lines check if the other UPS-down-file has been created ; if no, the down-file is created for the ups being down, and exits. If yes, meaning the other UPS is down, this one is getting down as well, thus the script should continue and shutdown the machine.
The files /tmp/usp[01].is.down
indicate if the ups[01] is currently down.
Important: the init.d start script of apcupsd should remove these files, if they have been created in a previous session:
rm -f /tmp/usp[01].is.down
Finally, the directories created above, /etc/apcupsd/ups[01] should be given access to the apcupsd user (or to whichever relevant user running the instances).
chown -R apcupsd /etc/apcupsd/ups[01]
Please have a look at the detailed documentation.
edit fixed the /tmp/ups[01].is.down
names, the .is
was missing.
Best Answer
I had a warranty PSU exchange in maybe 30% of my devices over the years. I consider this high enough that I want redundancy for this and would recommend it wherever feasible. In fact, PSUs were the only part except HDs I ever had to replace.
There are other reasons to consider redundant power as well: