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

From: Zdenek Kabelac <zkabelac@redhat.com>
To: device-mapper development <dm-devel@redhat.com>,
	Mikulas Patocka <mpatocka@redhat.com>
Cc: Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>,
	Mike Snitzer <snitzer@redhat.com>,
	Mandeep Baines <msb@chromium.org>,
	linux-kernel@vger.kernel.org, Mark Salyzyn <salyzyn@google.com>
Subject: Re: [dm-devel] [PATCH 0/4] dm verity: add support for error correction
Date: Fri, 6 Nov 2015 20:20:15 +0100	[thread overview]
Message-ID: <563CFD6F.4040009@redhat.com> (raw)
In-Reply-To: <20151106190634.GA2813@google.com>

Dne 6.11.2015 v 20:06 Sami Tolvanen napsal(a):
> 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.

AFAIK - you just build as much partition as need  to have some 'space'
dedicated for redundancy -  and rest of  'partitions' you join as 'writable'

i.e. you have 1G of space - you want to give 250MB as 'redundancy' -
so create 4 partition....

Since phones starts to have 64GB of storage space - it looks like a way to go.....

As no one bothers to upgrade 'old' phone - why to focus there??

And BTW - already seen couple bricked  Nexus7....
(And no marhmallow in plan....)
(And interestingly all killed I've seen were on Lolipop - none with Kitkat)

Zdenek