linux-ext4 - Re: [PATCH 2/3] ext4: Context support

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Message-Id: <201206160649.57838.arnd.bergmann@linaro.org>
Date:	Sat, 16 Jun 2012 06:49:57 +0000
From:	Arnd Bergmann <arnd.bergmann@...aro.org>
To:	"Ted Ts'o" <tytso@....edu>
Cc:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Alex Lemberg <Alex.Lemberg@...disk.com>,
	HYOJIN JEONG <syr.jeong@...sung.com>,
	Saugata Das <saugata.das@...aro.org>,
	Artem Bityutskiy <dedekind1@...il.com>,
	Saugata Das <saugata.das@...ricsson.com>,
	linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-mmc@...r.kernel.org, patches@...aro.org, venkat@...aro.org,
	"Luca Porzio (lporzio)" <lporzio@...ron.com>
Subject: Re: [PATCH 2/3] ext4: Context support

On Friday 15 June 2012, Ted Ts'o wrote:
> On Fri, Jun 15, 2012 at 09:19:23AM +0000, Arnd Bergmann wrote:
> > 
> > The trouble is that detecting the erase block size requires us to
> > write specific patterns to the device, which is generally a bad
> > idea after the file system has been created.
> 
> How much space do you need?  It's not hard to allocate a bunch of
> space, in a file, use FIEMAP ioctl to verify that you have a
> contiguous range of blocks, and then do direct I/O into that region.

We need a few erase blocks, spaced apart by a few erase blocks each.
Since we don't have to detect the number of erase blocks that the
device can handle, a small number would be ok I guess, so in order
to detect an 8 MB erase block correctly, we might use 3 erase blocks
that are spaced apart by 5 erase blocks, for a total of 104MB.

Once we figure out the erase block size, it would also help to
verify that we can write to at least e.g. 5 blocks concurrently
without triggering garbage collection, so we can print a warning
if it doesn't.

	Arnd
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