Difference: Ups (3 vs. 4)

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Nevis UPS Management

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The network switches, including the firewall in the network room, are also attached to UPSes. If a system's network is connected to a switch whose UPS goes critical, NUT will shut down the system.
The idea is if a system loses its network connectivity, odds are that its NIS and automount services will get into a bizarre state that would delay or prevent the completion of a shutdown. It's best to issue the shutdown command before that occurs.

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Some UPSes supply power to more than one system; as of May-07, an example of this is that both lincoln and sullivan are plugged into the same UPS. In such a situation, one system is the "UPS master" and the other is the "UPS slave"; NUT on the master usually communicates directly with the UPS, while the slave gets the UPS status by communicating with the master. If the UPS goes critical, the slave will shutdown immediately; the master will wait a minute or so to give a chance for the slave to receive the critical signal.

Some UPSes communicate their status via serial cables, which can only be connected to a single system; that's the reason for the "master-slave" situation described in the previous point.

The rest of the UPSes have SNMP management cards attached, which communicate their status via the ethernet. This has two advantages:

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Allt of the UPSes have SNMP management cards attached, which communicate their status via the ethernet.

- More than one system can monitor the UPS status simultaneously. This is especially useful if the UPS supplies power to a network switch, as noted above.
- It allows a systems administrator to reboot a system remotely, by shutting off the UPS's power via the management card. This has already proved useful when a system has gotten into a state in which it was not possible to log in; the system can be rebooted without waiting for the sysadmin to travel to Nevis.
  For this reason, all the major servers (including the mail server) have UPSes with SNMP management cards.

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The BIOS on all the systems has been set to automatically start the system back up on AC power restore. If that was not set, then the system would remain off even after Nevis power came back on and the UPS began supplying power to the system again.

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Some systems have an older BIOS that cannot be set to automatically start on AC power restore; polaris.nevis.columbia.edu is an example. The BIOS on those systems was fixed at the factory to go to the "last state": if the system was powered down normally, then when AC power is restored the system will remain down. On such systems, NUT has been configured to not issue a system shutdown when the attached UPS goes critical. So these "old-BIOS" systems run until their battery runs out of power, then crash; they come up immediately when the UPS starts providing power again.
This is a risk; the point of a UPS and NUT is to help machines shut down and start up cleanly. However, it turns out the delays caused by waiting for a systems administrator to give such systems personal attention outweigh the risk.

Some UPSes (e.g., the one that supplies power to the mail server) do not turn on their power immediately after Nevis power is restored; they are set to delay a few minutes. The reason is that those systems will come up more smoothly if other Nevis systems are already on; this is the case for the mail server, which mounts a lot of directories from other systems.

Once a week, hypatia.nevis.columbia.edu sends a command to each UPS to test its status. Once a month, hypatia sends a command to calibrate each UPS' battery under its current load. These tests are run in the early-morning hours, between 2AM and 5AM.
Aside from keeping the UPS status page accurate, these tests help assure us that the UPS batteries are functioning properly. Typically, a UPS battery has to be replaced about once every five years; these tests let us know when it's time for a replacement.

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