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Why Power Failures Can Always Lead To Data Loss
bigsmoke writes "So, all your servers run on RAID. You back up religiously. You're even sure that your backups are recoverable. But do you also need a UPS? According to Halfgaar (on Slashdot before to promote better Linux backup practices), yes, usually you do. He argues that despite technological advancements such as file system journaling, power failures can still cause data loss in most setups."
This is old hat in embedded systems.
Yes, but embedded systems usually have lower power requirements, or at the very least, a smaller range of power requirements. You can't add 3 PCIe cards, a few extra drives, and a few more GB of RAM to most embedded systems.
I worked on the design of an embedded system a few years ago that had a holdup spec - I think it was supposed to survive for 50 ms with no power. So a 50 ms power interruption would result in continued operation, while an outage longer than that was allowed to reset the board. However, the power draw on the board was around 200 Watts; being able to supply that much power for that long in a fairly compact form factor was a huge hurdle. It also caused airflow problems, because the giant capacitors would prevent air from getting to other components on the board, like the CPU. In the next version of the spec, I believe the holdup requirement was eliminated - apparently we weren't the only ones having trouble meeting that requirement.
Rule #1.
NEVER plug a laser printer into a UPS. The power that the fuser draws is WAY too much.
Look at some of the cheap office units, they show little pictures on them, notice the printer icon is on the surge side, NOT battery/surge side.
If the power goes out, you should NOT be trying to print.
http://articles.techrepublic.com.com/5100-10878_11-6085460.html See #6
http://arstechnica.com/guides/other/ups.ars/3
http://www.jetcafe.org/npc/doc/ups-faq.html#0405 see 04.05
Would you put a space heater on a UPS? Shredder? Vacuum? Table Saw? If you put a laser printer on it, you may as well.
Do not meddle in the affairs of sysadmins, for they are subtle, and quick to anger.
This reminds me of my favorite power loss story. The facility was doing a generator test, where we were supposed to switch over from city power to the generator. Unfortunately it didn't happen smoothly and the UPS kicked in. Sadly it turned out that so many servers had been added since the original design, the UPS was really only good for fifteen minutes or so. The final problem was that our operator didn't notice the issue quickly enough and so the next thing everyone in IT knew is that our main data center just lost power.
We spent most of the day getting our servers back up from various states of disrepair (confirming the article, power loss is superbad). It turns out that our main medical software ran on a Tandem. Though the drives and such lost power, the CPU had a backup of D-batteries and survived the power loss just fine. Needless to say, we stopped making fun of their seemingly primitive emergency backup power.