UBIFS recovery fails

[Jean-Sébastien Gagnon]

Hi, 
Actually, I think the empty space corruption is the only thing to address in this 
Specific problem, since any other error cause by unstable bits on valid data should be 
corrected by the parities in the flash driver.

This problem could be addressed by the NAND driver reading the pages and correcting 
empty ones, or by correcting UBIFS to address this problem.


J-S Gagnon

>
> On Mon, 2011-10-17 at 15:29 +0200, Daniel Kuhn wrote:
>> Hi,
>> 
>> I have a problem with a device which uses UBI + UBIFS on a 32GB NAND 
>> Flash (16*2GB). The
>> filesystem worked without problems for a couple of months but now I get 
>> an error if I try to mount the volume.
>> Attaching the UBI-Device works fine as you can see in the following 
>> messages:
>
> This issue looks like one of the MLC-specific ones. Unfortunately,
> no one really invested time into making UBIFS support MLC very well.
> It needs some more work. It also have some issues related to unstable
> bits in modern SLC.
>
> In short - if you want to use UBIFS on MLC - you should not have unclean
> reboots. If you want to make UBIFS 100% uclean-reboot save on MLC - you
> need to work on it some more.
>
> We (the original authors) developed and tested it on very robust SLC
> NAND.
> 
> Unfortunately, I do not work with MTD for few years already and have
> no spare time to make it MLC-robust. My last attempt was this spring -
> I started making integck test (in mtd-utils) support emulated power
> cuts. The idea was to improve UBIFS power-cut emulation infrastructure
> and make it emulating unstable bits, and then test and fix all issues.
> But then I realized that I simply will not have time to finish it,
> so left the work half-done.
>
> If someone wants to see UBIFS 100% or near 100% power-cut safe on
> MLC or one of shitty modern SLCs - he needs to invest men-hours.
> I can help by suggesting and reviewing. Although the funny thing is
> that eMMCs die and lose data in case of power cuts very often :-)
>
>> UBI: wear-leveling threshold:    4096
> Are you sure it is good for MLC? What is your eraseblock life-cycle?
>
>> Is there a way to recover the data on the device?
>
> If you just have precious data which you need - I think:
> 1. Make a dump of the flash.
> 2. Verify that on another device you can flash it and reproduce this
> issue.
> 3. Just hack the function in kernel that fails to return 0 instead
> of error for that case, and I think you will be able to mount your
> flash and copy your data.
>
> But if you want this to never happen again - you need to prepare
> for a several months project (providing you have a good kernel
> engineer).
>
> HTH :-)



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