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SubjectRe: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
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"H. Peter Anvin" <hpa@zytor.com> wrote on 01/18/2008 07:08:30 AM:

> 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.
> >
>
> 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.

I believe the power for that comes from a third source: a spring. Parking
the heads is too important to leave to active circuits.

--
Bryan Henderson IBM Almaden Research Center
San Jose CA Filesystems



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