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Subject: Re: bbt and bitflip
From: Artem Bityutskiy <dedekind1@gmail.com>
To: Brian Norris <computersforpeace@gmail.com>
Date: Fri, 24 Jun 2011 22:55:36 +0300
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On Thu, 2011-06-23 at 09:36 -0700, Brian Norris wrote:
> I am interested in Artem's comments on the robustness of flash-based
> BBT (here, and more recently on
> http://lists.infradead.org/pipermail/linux-mtd/2011-June/036557.html).
> I recently have moved to using flash-based BBT (in-band, actually),
> and it seemed like several NAND drivers use flash-based BBT as well.
> Is it really that un-trustworthy?

If you confirm that everything is great and robust when the on-flash BBT
gets corrupted - the NAND core for sure notices any corruptions and
falls-back to the traditional scanning method and restores the on-flash
BBT - then I apologize for saying that I do not trust it. Also, I do not
really know the details of this, so I may be completely wrong.

> "The bbt should be protected with CRC and if it gets corrupted we
> should re-scan the flash and re-create it."
> 
> Wouldn't CRC just be a lesser replacement for proper ECC protection?
> Or am I missing something?

I'd say ECC and CRC play different roles. ECC is about handling NAND
PITAs like read/write/erase disturb, and CRC is about noticing any
corruption and recover, instead of reporting inaccurate information up -
e.g., reporting a good block as bad.

-- 
Best Regards,
Artem Bityutskiy (Битюцкий Артём)