On Mon, Sep 12, 2022 at 5:22 PM Miklos Szeredi <miklos@xxxxxxxxxx> wrote: > > On Mon, 12 Sept 2022 at 15:26, Amir Goldstein <amir73il@xxxxxxxxx> 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.
