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

[Amir Goldstein <amir73il@gmail.com>]

[+overlayfs list]

On Mon, Feb 6, 2023 at 7:16 PM Amir Goldstein <amir73il@gmail.com> wrote:
>
> On Mon, Feb 6, 2023 at 6:34 PM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Mon, 6 Feb 2023 at 14:31, Amir Goldstein <amir73il@gmail.com> wrote:
> > >
> > > > > > 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
> > > >
> > > > Sorry about being late to the party...
> > > >
> > > > Can you give a little detail about what exactly this does?
> > > >
> > >
> > > Consider a container image distribution system, with base images
> > > and derived images and instruction on how to compose these images
> > > using overlayfs or other methods.
> > >
> > > Consider a derived image L3 that depends on images L2, L1.
> > >
> > > With the composefs methodology, the image distribution server splits
> > > each image is split into metadata only (metacopy) images M3, M2, M1
> > > and their underlying data images containing content addressable blobs
> > > D3, D2, D1.
> > >
> > > The image distribution server goes on to merge the metadata layers
> > > on the server, so U3 = M3 + M2 + M1.
> > >
> > > In order to start image L3, the container client will unpack the data layers
> > > D3, D2, D1 to local fs normally, but the server merged U3 metadata image
> > > will be distributed as a read-only fsverity signed image that can be mounted
> > > by mount -t composefs U3.img (much like mount -t erofs -o loop U3.img).
> > >
> > > The composefs image format contains "redirect" instruction to the data blob
> > > path and an fsverity signature that can be used to verify the redirected data
> > > content.
> > >
> > > When composefs authors proposed to merge composefs, Gao and me
> > > pointed out that the same functionality can be achieved with minimal changes
> > > using erofs+overlayfs.
> > >
> > > Composefs authors have presented ls -lR time and memory usage benchmarks
> > > that demonstrate how composefs performs better that erofs+overlayfs in
> > > this workload and explained that the lookup of the data blobs is what takes
> > > the extra time and memory in the erofs+overlayfs ls -lR test.
> > >
> > > The lazyfollow POC optimizes-out the lowerdata lookup for the ls -lR
> > > benchmark, so that composefs could be compared to erofs+overlayfs.
> >
> > Got it, thanks.
> >
> > >
> > > To answer Alexander's question:
> > >
> > > > Cool. I'll play around with this. Does this need to be an opt-in
> > > > option in the final version? It feels like this could be useful to
> > > > improve performance in general for overlayfs, for example when
> > > > metacopy is used in container layers.
> > >
> > > I think lazyfollow could be enabled by default after we hashed out
> > > all the bugs and corner cases and most importantly remove the
> > > POC limitation of lower-only overlay.
> > >
> > > The feedback that composefs authors are asking from you
> > > is whether you will agree to consider adding the "lazyfollow
> > > lower data" optimization and "fsverity signature for metacopy"
> > > feature to overlayfs?
> > >
> > > If you do agree, the I think they should invest their resources
> > > in making those improvements to overlayfs and perhaps
> > > other improvements to erofs, rather than proposing a new
> > > specialized filesystem.
> >
> > Lazy follow seems to make sense.  Why does it need to be optional?
>
> It doesn't.
>
> > Does it have any advantage to *not* do lazy follow?
> >
>
> Not that I can think of.
>
> > Not sure I follow the fsverity requirement.  For overlay+erofs case
> > itsn't it enough to verify the erofs image?
> >
>
> it's not overlay{erofs+erofs}
> it's overlay{erofs+ext4} (or another fs-verity [1] supporting fs)
> the lower layer is a mutable fs with /objects/ dir containing
> the blobs.
>
> The way to ensure the integrity of erofs is to setup dm-verity at
> erofs mount time.
>
> The way to ensure the integrity of the blobs is to store an fs-verity
> signature of each blob file in trusted.overlay.verify xattr on the
> metacopy and for overlayfs to enable fsverity on the blob file before
> allowing access to the lowerdata.
>

Perhaps I should have mentioned that the lower /objects dir, despite
being mutable (mostly append-only) is shared among several overlays.

This technically breaks the law of no modification to lower layer,
but the /objects dir itself is a whiteout in the metadata layer, so
the blobs are only accessible via absolute path redirect and there
is no /objects overlay dir, so there is no readdir cache to invalidate.
Naturally, the content addressable blobs are not expected to be
renamed/unlinked while an overlayfs that references them is mounted.

Thanks,
Amir.