[RFC] initoverlayfs - a scalable initial filesystem

[Nils Kattenbeck]

On Tue, Dec 12, 2023 at 10:02 PM Lennart Poettering
<lennart at poettering.net> wrote:
>
> If you have 7 cpio initrds then the kernel will allocate a tmpfs and
> unpack them all into it, one after the other, on top of each other,
> and then jumps into the result.
>
> if you have an erofs and 7 cpio initds, what are you going to do? You
> cannot extract into an erofs, it's immutable. You'd need something
> like overlayfs, but that would require (at least for now) an
> additional step in userspace, which is something to avoid.
>
> Alternatively (and preferred by me) would support a mode where it
> would unpack any cpios it gets into a tmpfs, and then pass an fsopen()
> fd to that to the executable it then invokes from the erofs. the
> executable could then mount that somewhere if it wants. But this would
> require a kenrel patch.

Such a kernel patch would likely be the more advanced method.
I also saw that they now wrote to the LKML to potentially discuss
something like this.
The method with an overlaysfs would likely be easier for init systems
to use but also less customizable.

> > Even if everything is the same there are codes paths which might not
> > be taken during usual operation. An example would be services similar
> > to the new systemd-bsod which are only triggered in emergencies.
> > Having these in the cpio means that they will always be read and
> > decompressed.
>
> systemd-bsod is tiny though, less than 8K compressed here. Not sure it
> is a good example.

Yes that is right though it is the first and most universal thing
which came to mind.
A better example would be something like a fleet management SDK (in
Java or a similar language with a runtime) which phones to a
management server indicating a boot failure and publishing crash logs.

> > Using sysexts also has the drawback that each and every one of them
> > has to be decompressed. I might be mistaken but I expect that this
> > will be the case even if the extension-release in the sysext results
> > in it being discarded which is obviously another big drawback.
>
> sysexts are erofs or squashfs file systems with verity backing. Only
> the sectors you access are decompressed.

Okay I forgot that they were erofs based and mentioned cpio archives
so I assumed they would be one.
Do they need to be fully read from disk to generate the cpio archive?

> Lennart
>
> --
> Lennart Poettering, Berlin