writing garbage when the voltage drops

Posted Jul 15, 2006 5:21 UTC (Sat) by roelofs (guest, #2599)
In reply to: writing garbage when the voltage drops by giraffedata
Parent article: Crash-only software: More than meets the eye

I can easily believe that the motor generating power is fantasy, but I always assumed there was a capacitor in there that could supply enough energy to finish writing the current sector. Why wouldn't there be?

Size, maybe? I'm just shooting the breeze here, but caps associated with power supplies tend to be immensely bigger than typical hard-drive components, and I'd guess that one capable of acting as a power-supply standin for even a few milliseconds would still be quite a bit bigger than the little surface-mount discretes used on drives today.

But maybe I'm suffering from cranio-rectal impaction again... I hate it when that happens.

Greg

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writing garbage when the voltage drops

Posted Jul 17, 2006 14:29 UTC (Mon) by giraffedata (guest, #1954) [Link] (1 responses)

OK, I did some calculations. I think the drive needs less than 10 microseconds to finish writing a sector. In that time, it needs up to 1 ampere, and can work with at least 4v out of the 5v power supply. So a 10uF capacitor, which is the size of a pea, should suffice.

The stored energy in the disk probably is relevant too, in that it keeps the disk spinning fast enough for an acceptable write 10 uS after the motor loses power.

writing garbage when the voltage drops

Posted Jul 25, 2006 3:59 UTC (Tue) by barrygould (guest, #4774) [Link]

I'd expect you want clusters, not sectors, ensured to be written safely.