Re: [PATCH RESEND V12 3/8] fuse: Definitions and ioctl for passthrough

[Amir Goldstein <amir73il@gmail.com>]

On Mon, Sep 12, 2022 at 5:22 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
>
> On Mon, 12 Sept 2022 at 15:26, Amir Goldstein <amir73il@gmail.com> wrote:
>
> > FWIW duplicate page cache exists in passthough FUSE whether
> > passthrough is in kernel or in userspace, but going through yet another
> > "switch" fs would make things even worse.
>
> I imagine the "switch" layer for a HSM would be simple enough:
>
> a) if file exists on fastest layer (upper) then take that
> b) if not then fall back to fuse layer (lower) .
>
> It's almost like a read-only overlayfs (no copy up) except it would be
> read-write and copy-up/down would be performed by the server as
> needed. No page cache duplication for upper, and AFAICS no corner
> cases that overlayfs has, since all layers are consistent (the fuse
> layer would reference the upper if that is currently the up-to-date
> one).

On recent LSF/MM/BPF, BPF developers asked me about using overlayfs
for something that looks like the above - merging of non overlapping layers
without any copy-up/down, but with write to lower.

I gave them the same solution (overlayfs without copy-up)
but I said I didn't know what you would think about this overlayfs mode
and I also pointed them to the eBPF-FUSE developers as another
possible solution to their use case.

>
> readdir would go to the layer which has the complete directory (which
> I guess the lower one must always have, but the upper could also).
>
> I'm probably missing lots of details, though...
>

That's what I said too :)

Does that mean that you are open to seeing patches for
an overlayfs mode that does not copy-up on write to lower?
I can come up with some semantics for readdir that will
make sense.

Thanks,
Amir.