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

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Message-ID: <478FE22D.9030907@emc.com>
Date:	Thu, 17 Jan 2008 18:18:05 -0500
From:	Ric Wheeler <ric@....com>
To:	Theodore Tso <tytso@....edu>,
	Daniel Phillips <phillips@...gle.com>,
	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>, Rik van Riel <riel@...hat.com>,
	Valerie Henson <val.henson@...il.com>
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental
 fsck)

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.

> 
> The problem isn't with the disk drive; it's from the DRAM, which tend
> to be much more voltage sensitive than the hard drives --- so it's
> quite likely that you could end up DMA'ing garbage from the memory.
> In fact the fact that the disk drives lasts longer due to capacitors
> on the board, rotational inertia of the platters, etc., is part of the
> problem.

I can tell you directly that when you drop power to a drive, you will 
lose write cache data if the write cache is enabled. With barriers 
enabled, our testing shows that file systems survive power failures 
which routinely caused corruption without them ;-)

ric


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