linux-kernel - Re: [PATCH 0/4] dm verity: add support for error correction

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Date:	Fri, 6 Nov 2015 19:06:34 +0000
From:	Sami Tolvanen <samitolvanen@...gle.com>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Milan Broz <mbroz@...hat.com>,
	device-mapper development <dm-devel@...hat.com>,
	Mandeep Baines <msb@...omium.org>,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	Mike Snitzer <snitzer@...hat.com>,
	linux-kernel@...r.kernel.org, Alasdair Kergon <agk@...hat.com>,
	Mark Salyzyn <salyzyn@...gle.com>
Subject: Re: [PATCH 0/4] dm verity: add support for error correction

On Fri, Nov 06, 2015 at 12:23:29PM -0500, Mikulas Patocka wrote:
> I'm also wondering what is this patch useful for. Disks and flash 
> controllers have their own error detection and correction

I think I addressed this earlier. Some storage devices are able to
correct bit flips, but don't have enough redundancy to correct larger
errors. Using this patch set we can correct N MiB of consecutive
corruption anywhere on the partition with the same amount of storage
overhead.

> Another point - if the read-only system partition is experiencing some
> errors, than the read-write partition will probably have errors too

On mobile devices, errors in read-only partitions often lead to
bricked devices while errors in the read-write parts might only lead
to lost cat photos. There are situations where people would prefer to
have a working phone even if it fails to store some of their data.

> Do you have some real case where such error corrections 
> increase longevity of some device?

Yes, there have been several cases where read-only partition errors
have rendered a device unusable. The sheer volume of mobile devices
means that even if a tiny fraction of them suffer from such a problem,
it's going to affect a large number of people.

> But you can take raid5 in read-only mode, put it on several partitions 
> protected with dm-verity and you get decent error correction

I agree. Unfortunately, we don't currently have the luxury of using
raid on mobile devices.

Sami
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