linux-kernel - Re: ATA 4 KiB sector issues.

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Message-ID: <4B954BF5.2050506@zytor.com>
Date:	Mon, 08 Mar 2010 11:11:49 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	James Bottomley <James.Bottomley@...e.de>
CC:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Tejun Heo <tj@...nel.org>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Daniel Taylor <Daniel.Taylor@....com>,
	Jeff Garzik <jeff@...zik.org>, Mark Lord <kernel@...savvy.com>,
	tytso@....edu, hirofumi@...l.parknet.co.jp,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, irtiger@...il.com,
	Matthew Wilcox <matthew@....cx>, aschnell@...e.de,
	knikanth@...e.de, jdelvare@...e.de
Subject: Re: ATA 4 KiB sector issues.

On 03/08/2010 10:58 AM, James Bottomley wrote:
>>
>> On the flipside, though, there really is very little net benefit to 4K
>> as opposed to 512 byte logical sectors: the additional protocol overhead
>> is relatively minimal, and as long as writes are aligned full blocks,
>> there shouldn't be any additional overhead on either the OS or the drive
>> side.  On the plus side, you get full compatibility with the existing
>> software stack.  The equation really seems rather simple.
> 
> There's another problem that afflicts 4k drives emulating 512b: they
> have to do a read modify write for any isolated 512b write ... that
> leads to potential corruption of adjacent 512b blocks if power is lost
> at the moment the write is being done.  Since most Linux filesystems are
> 4k sectors, misalignment really hammers this, plus most journal writes
> seem to be done in 512 byte increments.  I suppose for USB this could be
> regarded as flakey as usual, though.
> 

Misalignment sucks in general.  This is nothing new - the RAID and flash
people have had these problems for a long time now.  It's clear we need
to align our filesystems, period.

As to the read-modify-write issue: to some degree there is very little
you can do about it other than a big enough capacitor.  If you can't
write a sector atomically and have it stick, you're screwed no matter what.

	-hpa
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