"H. Peter Anvin" <[EMAIL PROTECTED]> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
