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

From: Mikulas Patocka
Date: Sat Nov 07 2015 - 10:18:13 EST

On Fri, 6 Nov 2015, Sami Tolvanen wrote:

> 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.

So, why doesn't the patch correct I/O errors? It is more likely that the
flash controller returns an I/O error than corrupted data. Why are you
correcting corrupted data (that is unlikely) and not correcting I/O errors
that are likely?

> > 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.

The read-write partition holds compiled applications and I doubt the
smartphone would work if there were random bits flipped.

> > 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.

Why don't you reflash the device from bootloader? (by holding power and
volume keys simultaneously on startup and using the fastboot utility)

> > 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.

Why not, it's just a simple kernel option. If raid5 already does data
correction, there is no reason why to duplicate this work.

> Sami

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