Re: [PATCH v3 0/6] Composefs: an opportunistically sharing verified image filesystem

[Amir Goldstein]

> >>> Apart from that, I still fail to get some thoughts (apart from
> >>> unprivileged
> >>> mounts) how EROFS + overlayfs combination fails on automative real
> >>> workloads
> >>> aside from "ls -lR" (readdir + stat).
> >>>
> >>> And eventually we still need overlayfs for most use cases to do
> >>> writable
> >>> stuffs, anyway, it needs some words to describe why such < 1s
> >>> difference is
> >>> very very important to the real workload as you already mentioned
> >>> before.
> >>>
> >>> And with overlayfs lazy lookup, I think it can be close to ~100ms or
> >>> better.
> >>>
> >>
> >> If we had an overlay.fs-verity xattr, then I think there are no
> >> individual features lacking for it to work for the automotive usecase
> >> I'm working on. Nor for the OCI container usecase. However, the
> >> possibility of doing something doesn't mean it is the better technical
> >> solution.
> >>
> >> The container usecase is very important in real world Linux use today,
> >> and as such it makes sense to have a technically excellent solution for
> >> it, not just a workable solution. Obviously we all have different
> >> viewpoints of what that is, but these are the reasons why I think a
> >> composefs solution is better:
> >>
> >> * It is faster than all other approaches for the one thing it actually
> >> needs to do (lookup and readdir performance). Other kinds of
> >> performance (file i/o speed, etc) is up to the backing filesystem
> >> anyway.
> >>
> >> Even if there are possible approaches to make overlayfs perform better
> >> here (the "lazy lookup" idea) it will not reach the performance of
> >> composefs, while further complicating the overlayfs codebase. (btw, did
> >> someone ask Miklos what he thinks of that idea?)
> >>
> >
> > Well, Miklos was CCed (now in TO:)
> > I did ask him specifically about relaxing -ouserxarr,metacopy,redirect:
> > https://lore.kernel.org/linux-unionfs/20230126082228.rweg75ztaexykejv@wittgenstein/T/#mc375df4c74c0d41aa1a2251c97509c6522487f96
> > but no response on that yet.
> >
> > TBH, in the end, Miklos really is the one who is going to have the most
> > weight on the outcome.
> >
> > If Miklos is interested in adding this functionality to overlayfs, you are going
> > to have a VERY hard sell, trying to merge composefs as an independent
> > expert filesystem. The community simply does not approve of this sort of
> > fragmentation unless there is a very good reason to do that.
> >
> >> For the automotive usecase we have strict cold-boot time requirements
> >> that make cold-cache performance very important to us. Of course, there
> >> is no simple time requirements for the specific case of listing files
> >> in an image, but any improvement in cold-cache performance for both the
> >> ostree rootfs and the containers started during boot will be worth its
> >> weight in gold trying to reach these hard KPIs.
> >>
> >> * It uses less memory, as we don't need the extra inodes that comes
> >> with the overlayfs mount. (See profiling data in giuseppes mail[1]).
> >
> > Understood, but we will need profiling data with the optimized ovl
> > (or with the single blob hack) to compare the relevant alternatives.
>
> My little request again, could you help benchmark on your real workload
> rather than "ls -lR" stuff?  If your hard KPI is really what as you
> said, why not just benchmark the real workload now and write a detailed
> analysis to everyone to explain it's a _must_ that we should upstream
> a new stacked fs for this?
>

I agree that benchmarking the actual KPI (boot time) will have
a much stronger impact and help to build a much stronger case
for composefs if you can prove that the boot time difference really matters.

In order to test boot time on fair grounds, I prepared for you a POC
branch with overlayfs lazy lookup:
https://github.com/amir73il/linux/commits/ovl-lazy-lowerdata

It is very lightly tested, but should be sufficient for the benchmark.
Note that:
1. You need to opt-in with redirect_dir=lazyfollow,metacopy=on
2. The lazyfollow POC only works with read-only overlay that
    has two lower dirs (1 metadata layer and one data blobs layer)
3. The data layer must be a local blockdev fs (i.e. not a network fs)
4. Only absolute path redirects are lazy (e.g. "/objects/cc/3da...")

These limitations could be easily lifted with a bit more work.
If any of those limitations stand in your way for running the benchmark
let me know and I'll see what I can do.

If there is any issue with the POC branch, please let me know.

Thanks,
Amir.