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

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Message-ID: <20250902-life-domestic-c341a8992cac@thorsis.com>
Date: Tue, 2 Sep 2025 09:37:17 +0200
From: Alexander Dahl <ada@...rsis.com>
To: Richard Weinberger <richard.weinberger@...il.com>
Cc: linux-mtd@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: State of the UBI fastmap feature

Hello Richard,

Am Tue, Sep 02, 2025 at 09:23:04AM +0200 schrieb Richard Weinberger:
> 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.

We are still using it in cost-sensitive products.  Those raw NAND
flash chips are a lot cheaper than eMMC or even NOR flashs of the same
size.

> 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?

Indeed.  Need to reduce boot time here, and UBI is attached twice.
Once in U-Boot to read the kernel image and then again in Linux.

Greets
Alex