Re: [RFC][PATCH] UBI: Make MTD_UBI_FASTMAP non-experimental

[Marek Vasut]

On 03/29/2017 10:04 PM, Richard Weinberger wrote:
> Jesper,
> 
> Am 29.03.2017 um 17:38 schrieb Jesper Nilsson:
>> MTD_UBI_FASTMAP has been set as experimental since it
>> was merged back in 2012.
>>
>> There hasn't been much change in the format,
>> so we can consider the feature stable and start
>> being careful about breaking the format.
>> (This is somewhat of a pre-requisite for anyone actually
>> using the feature in the real world and depending on it)
>>
>> Drop the experimental note and the warning text about
>> the on-flash format not being finalized.
> 
> I fully agree, we can drop this note. But we have to add another
> one.
> While Fastmap is a nice feature to speed-up the attach time it
> comes with a cost. It makes UBI less robust. I saw issues
> on NAND chips which misbehaved slightly where UBI was able to
> recover when using a full scan but not when Fastmap was used.
> The UBI full scan code is paranoid and can sort out problems
> very early, with Fastmap enabled you lose this valuable property.
> 
> So, users should enable Fastmap only when they absolutely need
> a very fast attach time and be very sure that the NAND works as
> expected.

So we should document this with a big fat warning and set fastmap to
default=n ?

-- 
Best regards,
Marek Vasut