linux-kernel - Re: State of the UBI fastmap feature

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Message-ID: <CAFLxGvzO2yszspJtd_A3BSTCRNSF+ts_5+2aMkNksxM48DYEwA@mail.gmail.com>
Date: Tue, 2 Sep 2025 09:23:04 +0200
From: Richard Weinberger <richard.weinberger@...il.com>
To: linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: State of the UBI fastmap feature

On Tue, Sep 2, 2025 at 8:00 AM Alexander Dahl <ada@...rsis.com> wrote:
> Hello everyone,
>
> after using UBI for almost 15 years now without fastmap, I'm currently
> experimenting with that feature.  The help text in Kconfig menu
> (drivers/mtd/ubi/Kconfig) still says "Experimental feature" and
> further down:
>
> > Important: this feature is experimental so far and the on-flash
> > format for fastmap may change in the next kernel versions
>
> Is this still the current state?  I can not remember seing any patches
> touching fastmap in the last time.  Are there plans to stabilize this?
> Will there be changes in the on-flash format?  Do folks even use this
> feature?  In production?

I don’t expect changes to the on-disk format, but the number of new raw NAND
devices being introduced is now close to zero. As a result, the user
base is very small,
and subtle bugs are likely to remain undiscovered.

Fastmap does have users, including in production,
but I don’t have concrete numbers. And I don't know their workload.
That said, Fastmap should work but I still anticipate performance and
runtime issues.

I guess your motivation is reducing the attach time?

-- 
Thanks,
//richard