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

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Date:	Fri, 15 Jun 2012 17:30:46 -0400
From:	Ted Ts'o <tytso@....edu>
To:	Arnd Bergmann <arnd.bergmann@...aro.org>
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 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.

> I think the best we can do is
> 
> * default to "unspecified" as before
> * if "unspecified", make the file system ask the block device. in
>   case of eMMC, that will usually be reliable
> * Add an option to mkfs and tunefs to hardcode a specific size for
>   users that know the size and can't rely on the blockdev reporting
>   it correctly to the file system.
> * Add an option to mkfs to autodetect the size for the drive it's
>   run on.

Well, I think we can do better; the question is whether or not it's
worth the effort.  It may not be....

				- Ted
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