Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

[Bryan Henderson <hbryan@us.ibm.com>]

"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