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

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Date:	Thu, 17 Jan 2008 16:31:48 -0800
From:	Bryan Henderson <hbryan@...ibm.com>
To:	Ric Wheeler <ric@....com>
Cc:	Al Boldi <a1426z@...ab.com>, Alan Cox <alan@...rguk.ukuu.org.uk>,
	David Chinner <dgc@....com>, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>,
	Daniel Phillips <phillips@...gle.com>,
	Rik van Riel <riel@...hat.com>, Theodore Tso <tytso@....edu>,
	Valerie Henson <val.henson@...il.com>
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

Ric Wheeler <ric@....com> wrote on 01/17/2008 03:18:05 PM:

> Theodore Tso wrote:
> > On Wed, Jan 16, 2008 at 09:02:50PM -0500, Daniel Phillips wrote:
> >> Have you observed that in the wild?  A former engineer of a disk 
drive
> >> company suggests to me that the capacitors on the board provide 
enough
> >> power to complete the last sector, even to park the head.
> >>
> 
> Even if true (which I doubt), this is not implemented.
> 
> A modern drive can have 16-32 MB of write cache. Worst case, those 
> sectors are not sequential which implies lots of head movement.

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.

Ted brought up the separate issue of the host sending garbage to the disk 
device because its own power is failing at the same time, which makes the 
integrity at the disk level moot (or even undesirable, as you'd rather 
write a bad sector than a good one with the wrong data).

--
Bryan Henderson                     IBM Almaden Research Center
San Jose CA                         Filesystems

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