Re: What's the usage of md-autodetect.c

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On Tue, Mar 7, 2023 at 7:00 PM Geoff Back <geoff@xxxxxxxxxxxxxxx> wrote:
>
> Hello,
>
> MD Autodetect allows for the assembly of raid-1 mirror filesystems
> in-kernel before userspace starts, without requiring the complexity and
> fragility of using an initrd and switch-root.
> It is still in widespread use and given that it has a wide use case in
> any system that wants fully resilient boot/root devices (which can be
> done even on EFI) is likely to remain essential.  While the various
> high-profile general purpose Linux distributions such as Red Hat and
> Ubuntu may use initrd by default, there are innumerable scenarios,
> particularly in embedded space, where initrd is never used but RAID may
> be and hence auto-detect is used to assemble the actual root file system.

Hi Geoff

Thanks for the explanation.

>
> Removing it would be a major regression in 'md' functionality.
>
> Yes, it requires CONFIG_MD and the appropriate RAID personality
> (typically CONFIG_MD_RAID1) to be set to 'y' in Kconfig.  It doesn't

raid1 can be loaded automatically. So it doesn't need to set
CONFIG_MD_RAID1 to y.

> (IIRC) get built if CONFIG_MD is set to M.  Changing the default for
> CONFIG_MD should not have any impact on this so long as the ability to
> set CONFIG_MD=y does not get disabled (which would also be a regression).

I'm a little confused here. If I understand right, for the os that
doesn't use initrd
and we still have the ability to set CONFIG_MD=y, so we can set it to
y and rebuild
the kernel. So the raid1 can be assembled by md auto-detect, right?

>
> If auto-detect were to be considered for removal then IMO it needs to go
> through the full kernel feature deprecation/removal life cycle - i.e.
> first it gets marked as deprecated in KConfig, then after a decent time
> interval (years?) the default for the option is changed, and only after
> that has all happened without causing problems, the code gets considered
> for removal.

Thanks for this information

Regards
Xiao

>
> Regards,
>
> Geoff.
>
> Geoff Back
> What if we're all just characters in someone's nightmares?
>
> On 07/03/2023 03:04, Xiao Ni wrote:
> > From the code of md-autodetect.c, it looks like it's used to create
> > the raid device
> > during boot. Now we use udev rules to assemble the raid. Do we still need it?
> > What's the usage of md-autodetect?
> >
> > And in Kconfig, it depends on md-raid as Y when building a kernel. If we change
> > the default to M, md-autodetect will not work anymore, right?
> >
> > Best Regards
> > Xiao
> >
>





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