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

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Message-ID: <47910D6E.3070801@garzik.org>
Date:	Fri, 18 Jan 2008 15:34:54 -0500
From:	Jeff Garzik <jeff@...zik.org>
To:	ric@....com
CC:	Theodore Tso <tytso@....edu>, Bryan Henderson <hbryan@...ibm.com>,
	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>,
	Valerie Henson <val.henson@...il.com>
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental
 fsck)

Ric Wheeler wrote:
> Theodore Tso wrote:
>> On Thu, Jan 17, 2008 at 04:31:48PM -0800, Bryan Henderson wrote:
>>> 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.
>>
>> Well, it would be impossible or at least very hard to see that in
>> practice, right?  My understanding is that drives do sector-level
>> checksums, so if there was a partially written sector, the checksum
>> would be bogus and the drive would return an error when you tried to
>> read from it.
> 
> There is extensive per sector error correction on each sector written. 
> What you would see in this case (or many, many other possible ways 
> drives can corrupt media) is a "media error" on the next read.

Correct.


> You would never get back the partially written contents of that sector 
> at the host.

Correct.


> Having our tools (fsck especially) be resilient in the face of media 
> errors is really critical. Although I don't think the scenario of a 
> partially written sector is common, media errors in general are common 
> and can develop over time.

Agreed.

	Jeff



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