Strange system lockups - kernel saying disk error

Michael Powell nightrecon at
Sun Jun 5 20:49:00 UTC 2011

perryh at wrote:
> Power supplies do fail occasionally, and not always in obvious
> ways such as failing to turn on at all.  The output voltages may
> be a little too high or too low, or they may be correct but with
> excessive ripple or electrical noise; or the supply may be just
> fine until a disk draws a current spike to move the arm rapidly.

I've seen a fair number or power supplies degrade somewhere around the 5 
year mark. Simple voltage checks with a VOM and its accuracy will usually 
still show the voltages as being correct. To see the ripple you'll need an 
oscilloscope. Excessive ripple can make a PC appear to have all kinds of 
intermittent hardware failures with little or no rhyme or reason. A degraded 
power supply will show large variations in ripple based on load. The largest 
load from hard drives is when they are first spinning up. Servers are 
commonly configured with the ability to spin up drives one at a time with a 
short delay in between. You won't usually find this on a desktop. 

Generally, this situation will develop more often on an old machine that had 
a 'barely enough' capacity power supply when new. Add 3 more hard drives, 
bigger video, etc and it was still just inside the envelope until enough 
time went by and the power supply got old. Since the most amps pulled by the 
hard drives occurs on power up you will see the ripple on  a 'scope look 
really ugly while this happens. The unseen danger here is that bits on the 
drive(s) can get scrambled until things settle down. You will know this 
happens when stuff goes wrong and fsck is needed to get the file system 
clean, and after cleaning and working again will do the same thing again at 
some future reboot.

Easiest way to look at this without a 'scope is to simply substitute a known 
good PSU of sufficient rating from a machine with no troubles. If all the 
random nonsense suddenly stops, you'll know. This is easiest for folks these 
days as those without an analog electronics background are unlikely to have 
an oscilloscope laying around. 

> It might be worth checking the fan mounted on the CPU heatsink if
> there is one, and the fan in the power supply (which ventilates the
> case as well as the power supply itself).

Aside from the fans themselves, dust buildup plugs heat sinks eventually 
drastically reducing their ability to get rid of heat. When you get to this 
stage blowing them out with canned air can work wonders. My 2 servers at 
home sit on the floor and need this about once a year.


More information about the freebsd-questions mailing list