Re: [PATCH] Export MDRAID bitmap on disk structure in UAPI header file

[Mariusz Tkaczyk <mtkaczyk@kernel.org>]

On Fri, 3 Jan 2025 09:14:30 +0800
Yu Kuai <yukuai1@huaweicloud.com> wrote:

> Hi,
> 
> 在 2025/01/02 19:48, Tomas Mudrunka 写道:
> >> Just curious, what you guys do for filesystems like ext4/xfs, and
> >> they just define the same structure in user-space tools.
> >>
> >> looks like your tool do support to create ext4 images, and it's
> >> using ext4's use-space tools directly. If it's true, do you
> >> consider to use mdadm directly?
> >>
> >> Thanks,
> >> Kuai  
> > 
> > Yes, we do use external tools when possible. It is not possible
> > with mdadm. Mdadm cannot create disk image of MD RAID array. Kernel
> > does this.  
> 
> I'm a bit confused here, if you mean metadata, I think it's mdadm that
> write init metadata to disk, the only exception is dm-raid.

I think it means that currently you have to create kernel (MD) raid
device (assemble using metadata) to have a chance creating image.
> 
> > We want/need purely userspace generator, so we don't have to care
> > about permissions, losetup, kernel-side mdraid runtime, etc... We
> > just want to generate valid image without involving kernel in any
> > way.  
> 
> I believe mdadm can do this, Mtkaczyk what do you think?

I agree. The right way is to incorporate it with mdadm.
We should create a volume image (data) without MD internals.

With that, we will have control on this functionality. Also, we will be
able to provide support for every metadata format.
> 
> The problem is that system service will recognize raid disks and
> assemble the array automatically, you might what to disable them.

I don't think we need to care. They goal is to not have and use MD
module so mdadm will fail to load personalities.
> 
> Thanks,
> Kuai
> 
> > I was using mdadm before switching to genimage and it adds
> > complexity of handling all the edge cases of kernel states.
> > Mkfs.ext4 can create image without involving kernel, mdadm cannot,
> > it always instructs kernel to create the metadata for it when
> > creating array.
> > 
> > In my opinion we should decide whether it makes sense for kernel to
> > export the structures in header file and either provide all of
> > them, or provide none. That might be valid reasoning to say every
> > userspace program should include its own definitions of structures.
> > But providing half does not make any sense.

Sorry, This is old application and some solutions are here for years-
they are working so nobody tried to change them. If you are looking for
challenges this software is full of them!

Thanks,
Mariusz