Re: RFC: detect and manage power cut on MLC NAND

From: Andrea Scian <rnd4@dave-tech.it>
To: Richard Weinberger <richard@nod.at>, dedekind1@gmail.com
Cc: mtd_mailinglist <linux-mtd@lists.infradead.org>
Subject: Re: RFC: detect and manage power cut on MLC NAND
Date: Wed, 11 Mar 2015 18:01:06 +0100	[thread overview]
Message-ID: <550074D2.1070406@dave-tech.it> (raw)
In-Reply-To: <55000637.1030702@nod.at>

Hi all,

and thanks for you feedback.
You can find my comments below.

Il 11/03/2015 10:09, Richard Weinberger ha scritto:
> Am 11.03.2015 um 10:05 schrieb Artem Bityutskiy:
>> On Wed, 2015-03-11 at 09:57 +0100, Richard Weinberger wrote:
>>> Hi!
>>>
>>> Am 11.03.2015 um 08:20 schrieb Artem Bityutskiy:
>>>> On Tue, 2015-03-10 at 13:51 +0100, Richard Weinberger wrote:
>>>>>> WDYT about this?
>>>>>> If it sounds reasonable is there any suggestion where to place such a code?
>>>>> Customers often use DYI uninterruptible power supplies using capacitors.
>>>>> But managing a power cut is the least problem you have with MLC NAND.
>>>> Why is it the least problem, what is the hardest one? I thought this one
>>>> is the hardest.
>>> IMHO the hardest ones are the problems we don't know yet as NAND vendors
>>> are not really chatty about the MLC constraints.
>>> We don't know much about data retention for example. At least we have not
>>> much hard facts. Most of our knowledge is hearsay.
>> Well, but from the problems we know paired pages seems to be the biggest
>> one. E.g., what do we do if VID header gets corrupted because of an
>> interrupted write to the page paired with the VID header page? Sounds
>> like a hard problem to me.
> It is a hard problem. But at least we know about it.
>

IIUC can I summarize like the following

1) power cut is one of the big problem we know about MLC NAND flash with UBI
2) we are unaware of other big issue with MLC NAND

Unfortunately I don't really know the MTD/UBI/UBIFS internals but
(correct me if I'm wrong) to me solving the power cut issue only by
software (by using proper data structure, redundancy, NAND flash
operation jornal and so on) is pretty hard to implement inside the Linux
MTD stack.

On the other hand, stuff like scrubbing (e.g. having a task like GC that
periodically scrub the required NAND erase block), paired page (e.g.
carefully selecting LEB to PEB mapping to avoid reading/writing data
into paired page in a wrong way) are still to be implemented but have,
somehow, less impact on the whole MTD stack.

For sure I'm missing some other MLC NAND issue, but the above are the
one that I'm aware of.

WDYT?

Best Regards,

-- 

Andrea SCIAN

DAVE Embedded Systems