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

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Date:	Thu, 14 Jun 2012 12:14:13 -0400 (EDT)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Ted Ts'o <tytso@....edu>
cc:	Arnd Bergmann <arnd.bergmann@...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 Wed, 13 Jun 2012, Ted Ts'o wrote:

> On Wed, Jun 13, 2012 at 08:43:47PM +0000, Arnd Bergmann wrote:
> 
> > For SD cards, almost everyone is lying and we cannot trust the
> > information, and for USB flash, there is no way to ask the device.
> > In both of these cases, we probably want to detect the erase block
> > size at mkfs time using some timing attack that I worked on before.
> > Note that those devices also do not offer support for context IDs.
> 
> Yes, although presumably aligning large files to erase block
> boundaries would still be useful, yes?
> 
> So adding an erase block size to the ext2/3/4 superblock sounds like a
> first great step.  By making it be a superblock field, that way it's
> possible to override the value returned by the hardware if it turns
> out to be a lie, and we can also use programs like flashbench to
> figure out the erase block size and populate the superblock value via
> some userspace process.  (Possibly called out of mke2fs directly if we
> can automate it completely, and make it dead reliable.)

Let's not forget that, in almost all cases, filesystem images are not 
created live on the final medium.  Factories are picking a batch of 
flash devices and a pre-built filesystem image is stamped on them, and 
there might not even be a guarantee that those flash devices will all 
have the same characteristics.  So to say that making this tuning at 
mkfs time is probably not the best strategy.


Nicolas
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