Messages in this thread | | | Date | Fri, 18 Jan 2008 10:08:30 -0500 | From | "H. Peter Anvin" <> | Subject | Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck) |
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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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