linux-ext4 - Re: Question about data integrity on SD cards

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Date:	Tue, 15 Jul 2014 09:09:36 +0200
From:	David Jander <david@...tonic.nl>
To:	Grzegorz Sikorski <g.sikorski@...vatek.com>
Cc:	Lukáš Czerner <lczerner@...hat.com>,
	linux-ext4@...r.kernel.org
Subject: Re: Question about data integrity on SD cards

On Wed, 09 Jul 2014 14:23:29 +0100
Grzegorz Sikorski <g.sikorski@...vatek.com> wrote:

> We are using 2.6.37 version and we may have some issues with underlying 
> mmc-host driver. We also tested our setup on newer kernels as well as 
> different media with similar results, but I was not involved in these 
> tests, so cannot give any details straight away.
> 
> But my main question was partially answered by you. I just need a 
> confirmation that design of our system should in general be OK, as we 
> may expect random power downs occasionally. So do you think it is 
> possible (in general) to build embedded system with SQLite3 database on 
> EXT4 filesystem on SD card that is not fragile on unexpected power 
> sequence? Do you know what type of SD card would be recommended in this 
> case? Are our mounting options fine?

Sorry for the late reaction. In case you still wonder, no. It is not possible
to do this with common (COTS) SD cards in general. Due to their nature, SD
cards can corrupt even data that is not being written to when power is
interrupted during a write (or read) action. There is no filesystem that can
protect you from that.

And btw, the thread Lukáš mentioned is about a bug in ext4, not really about
problems with SD cards.

If you need robust flash storage, please look at MMC 4.41+ enhanced mode and
"reliable-write" functionality. Without these two functions combined, it is
not possible to do what you want to do. I have not seen (non embedded) MMC
cards (in actual SD/MMC-card format) that support version 4.41 or newer of the
(e)MMC standard, but it may be possible that someone makes them.... that might be
a solution for your problem.

Best regards,

-- 
David Jander
Protonic Holland.
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