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Subject: Re: UBIFS question
To: Martin Townsend <mtownsend1973@gmail.com>
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From: Richard Weinberger <richard@nod.at>
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Date: Thu, 17 Mar 2016 12:25:48 +0100
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Martin,

Am 17.03.2016 um 12:16 schrieb Martin Townsend:
>>> 2) One thing I'm going to have to do is write a background thread to
>>> monitor the status of the filesystem and try and detect corruption
>>> before the system becomes unstable, is there any way to find out the
>>> validity of the LEBs, ie checking their checksums.
>>
>> So, what exactly is the error scenario you have in mind?
>> If the SLC NAND behaves correctly UBIFS can deal with all kinds
>> of errors.
>> Of course UBI (and UBIFS) is not a magic bullet, if a NAND block
>> turns bad all of a sudden there is nothing it can do for you.
>> But this NAND would also not be with in the spec...
>>
>> It is not clear do me what this background thread should achieve.
> 
> We expect the flash devices to start failing quicker than normally
> expected due to the environment in which they will be operating in, so
> sudden NAND blocks turning bad will eventually happen and what we
> would like to do is try and capture this as soon as possible.
> The boards are not accessible as they will be located in very remote
> locations so detecting these failures before the system locks up would
> be an advantage so we can report home with the information and fail
> over to the other filesystem (providing that hasn't also been
> corrupted).

Dealing with sudden bad NAND blocks is almost impossible.
Unless you have a copy of each block.
NAND is not expected to gain bad blocks without an indication like
correctable bitflips.

Thanks,
//richard