Graceful UPS Shutdowns on Linux
There's nothing pleasant about the data corruption or even hardware damage that can result from a power failure. Carla Schroder explores how UPS units and a couple of Linux UPS tools can help safeguard your mission-critical computers.
Power failures are a fact of life, and as diligent, conscientious network admins, we implement whatever power backups we can wrangle funding for. A number of bad things can happen with a sudden power interruption: data corruption, mangled file systems, and even hardware damage. At the very least, we need to give important systems a chance to shut down gracefully, and then start back up when the power returns.
This is exactly what a UPS – uninterruptible power supply – is designed for. The term "uninterruptible power supply" is a bit misleading, though, as there are actually two types of UPS: lower-cost models that provide a battery backup and higher-end models that supply power continuously. The lower-end products, which used to be called "SPS" (standby power supply) units, provide a battery backup that activates during a power interruption.
The higher-end models, on the other hand, supply power continuously, and are truly "uninterruptible." They also "condition" the power, smoothing out sags and spikes. The electricity that comes out of our outlets is rather grotty — it isn't the nice, clean, carefully controlled voltages that computers prefer; rather it arrives in lumps and bumps. While mostly unnoticed by the casual user, even little tiny sags and surges in electricity take their toll over time. A true UPS costs 50-100% more than its SPS cousins, and the batteries must be replaced more often.
Whatever type of UPS you decide to use, it will only be good for running a system for a limited period of time. Some of the high-end models claim up to 90 minutes, but 10-15 is more typical. Their most important feature, in my not-so-humble-opinion, is being able to perform a graceful, unattended shutdown.
All UPS units come with nice Windows software for doing this. Some even provide Linux management and configuration software, though as usual, beware of binary-only drivers; useless old, moldy drivers; and drivers restricted to specific Linux distributions. (Hint to vendors: time did not stop the day Red Hat 6.0 was released.)