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

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Message-ID: <563D1614.5010501@redhat.com>
Date:	Fri, 6 Nov 2015 22:05:24 +0100
From:	Zdenek Kabelac <zkabelac@...hat.com>
To:	device-mapper development <dm-devel@...hat.com>
Cc:	Will Drewry <wad@...omium.org>, Kees Cook <keescook@...omium.org>,
	Mike Snitzer <snitzer@...hat.com>,
	Mandeep Baines <msb@...omium.org>,
	linux-kernel@...r.kernel.org,
	Mikulas Patocka <mpatocka@...hat.com>,
	Mark Salyzyn <salyzyn@...gle.com>
Subject: Re: [dm-devel] [PATCH 0/4] dm verity: add support for error
 correction

Dne 6.11.2015 v 21:27 Sami Tolvanen napsal(a):
> On Fri, Nov 06, 2015 at 08:20:15PM +0100, Zdenek Kabelac wrote:
>> i.e. you have 1G of space - you want to give 250MB as 'redundancy' -
>> so create 4 partition....

well data safety has it's price - user should choose what he prefers
  - more games and videos or more safety...

>
> We cannot afford to set aside 25% of read-only partition space for
> redundancy on mobile devices, and would rather not impact performance
> any more than dm-verity already does. With error correction we have
> 0.8% space overhead in our use case and no performance degradation if
> the partition is not corrupted.
>

I'm probably missing here some hw knowledge here - but if you loose
a flash block of some size  - then you typically get  'error' for
all bytes the sector/block.

So how do you want to correctly 'restore'  missing full sectors  with just 
0.8% data overhead ??

Or is the device which fails to correct block returning something 'still 
usable'  (since e.g.  SATA disk certainly not)

Zdenek

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