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

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Date:	Mon, 08 Mar 2010 13:16:48 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
CC:	James Bottomley <James.Bottomley@...e.de>,
	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 12:19 PM, Martin K. Petersen wrote:
>>>>>> "hpa" == H Peter Anvin <hpa@...or.com> writes:
> 
> hpa> On the flipside, though, there really is very little net benefit to
> hpa> 4K as opposed to 512 byte logical sectors: the additional protocol
> hpa> overhead is relatively minimal, and as long as writes are aligned
> hpa> full blocks, there shouldn't be any additional overhead on either
> hpa> the OS or the drive side.  On the plus side, you get full
> hpa> compatibility with the existing software stack.  The equation
> hpa> really seems rather simple.
> 
> 4KB sectors are not a win for anybody except the drive vendors.
> 

Obviously.  However, larger physical storage unit sizes -- 4K for
spinning media, but frequently much larger for flash, for example -- is
already in wide use, and having a huge mishmash of logical block sizes
isn't going to work very well.

> There is a push in the industry right now to keep the 512-byte logical
> blocks forever.  The first step would be to report misaligned accesses
> or accesses that are not a multiple of the physical block size.  Second
> step would be to eventually reject any write that's not a properly
> aligned multiple of the physical block size.

I personally suspect that that is the way it is going to go, rather than
trying to change the software ecosystem to a different logical block
size.  It has been tried in the past and failed, with the sole exception
of CD-ROMs, pretty much.

	-hpa
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