Subject: Re: [RFC][PATCH] UBI: Make MTD_UBI_FASTMAP non-experimental
To: Richard Weinberger <richard@nod.at>,
        Jesper Nilsson <jesper.nilsson@axis.com>,
        Artem Bityutskiy <dedekind1@gmail.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Brian Norris <computersforpeace@gmail.com>,
        Boris Brezillon <boris.brezillon@free-electrons.com>,
        Cyrille Pitchen <cyrille.pitchen@atmel.com>,
        linux-mtd@lists.infradead.org, linux-kernel@vger.kernel.org
References: <20170329153836.GB29118@axis.com>
 <434195d8-d638-240d-8d63-50d033ea453a@nod.at>
From: Marek Vasut <marek.vasut@gmail.com>
Message-ID: <3f8d0417-5e7d-b7c8-ba83-9a87e774f97f@gmail.com>
Date: Thu, 30 Mar 2017 12:01:41 +0200
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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