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SubjectRe: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Bryan Henderson wrote:
>
> We weren't actually talking about writing out the cache. While that was
> part of an earlier thread which ultimately conceded that disk drives most
> probably do not use the spinning disk energy to write out the cache, the
> claim was then made that the drive at least survives long enough to finish
> writing the sector it was writing, thereby maintaining the integrity of
> the data at the drive level. People often say that a disk drive
> guarantees atomic writes at the sector level even in the face of a power
> failure.
>
> But I heard some years ago from a disk drive engineer that that is a myth
> just like the rotational energy thing. I added that to the discussion,
> but admitted that I haven't actually seen a disk drive write a partial
> sector.
>

Did he work for Maxtor, by any chance? :-/

A disk drive whose power is cut needs to have enough residual power to
park its heads (or *massive* data loss will occur), and at that point it
might as well keep enough on hand to finish an in-progress sector write.

There are two possible sources of onboard temporary power: a large
enough capacitor, or the rotational energy of the platters (an
electrical motor also being a generator.) I don't care which one they
use, but they need to do something.

-hpa



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