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Date:	Mon, 15 Mar 2010 10:56:05 +0100
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	david@...g.hm
Cc:	"H. Peter Anvin" <hpa@...or.com>, Arnd Bergmann <arnd@...db.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 Monday 15 March 2010 06:20, david@...g.hm wrote:
> >>> For any other partition, pick start cylinder which is a multiple of 8:
> >>>
> >>> cyl 8*x head 0 sector 1: LBA sector 8*x*255*63 - good (4k aligned)
> >>>
> >>> This will actually work well for *any* geometry, not only for 63s/255h.
> >>
> >> Yes, but it does squat for a flash disk that wants, say, 256K alignment.
> >
> > 4K makes sense. 256K not so much.
> >
> > 256K alignment is hard to swallow for a lot of reasons anyway.
> > Unless the filesystem packs small files into blocks a-la reiserfs,
> > 256K block filesystems will be very inefficient for a typical
> > storage scenarios.
> 
> the thing is, if the OS can learn that it's more efficiant to write in 
> 256K aligned chunks, then it can batch up things so that the drive doesn't 
> have to do a read-modify-write cycle and can instead just replace the 
> entire chunk.

I think Linux already is doing this. The problem is, in many cases
OS can't possibly do this, short of using a specially designed
filesystem.

If you untar a Linux kernel source tarball on a seriously
fragmented ext2 filesystem, there will be a lot of discontiguous
and/or misaligned writes smaller than 256K.
Only smart firmware can help in this case.
-- 
vda
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