In-Reply-To: <4790C0EE.10207@zytor.com>
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: Al Boldi <a1426z@gawab.com>, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       David Chinner <dgc@sgi.com>, linux-kernel@vger.kernel.org,
       Pavel Machek <pavel@ucw.cz>, Daniel Phillips <phillips@google.com>,
       Ric Wheeler <ric@emc.com>, Rik van Riel <riel@redhat.com>,
       Theodore Tso <tytso@mit.edu>, Valerie Henson <val.henson@gmail.com>
MIME-Version: 1.0
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)
Message-ID: <OFC2B46E42.BDE3F6A4-ON882573D4.0060B268-882573D4.00615C9C@us.ibm.com>
From: Bryan Henderson <hbryan@us.ibm.com>
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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