Messages in this thread | | | Subject | Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck) | From | Bryan Henderson <> | Date | Fri, 18 Jan 2008 09:43:25 -0800 |
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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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