Re: dm overlaybd: targets mapping OverlayBD image

From: Alexander Larsson
Date: Wed May 24 2023 - 02:47:04 EST

On Tue, May 23, 2023 at 7:29 PM Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> On Fri, May 19 2023 at 6:27P -0400,
> Du Rui <durui@xxxxxxxxxxxxxxxxx> wrote:
> > OverlayBD is a novel layering block-level image format, which is design
> > for container, secure container and applicable to virtual machine,
> > published in USENIX ATC '20
> >
> >
> > OverlayBD already has a ContainerD non-core sub-project implementation
> > in userspace, as an accelerated container image service
> >
> >
> > It could be much more efficient when do decompressing and mapping works
> > in the kernel with the framework of device-mapper, in many circumstances,
> > such as secure container runtime, mobile-devices, etc.
> >
> > This patch contains a module, dm-overlaybd, provides two kinds of targets
> > dm-zfile and dm-lsmt, to expose a group of block-devices contains
> > OverlayBD image as a overlaid read-only block-device.
> >
> > Signed-off-by: Du Rui <durui@xxxxxxxxxxxxxxxxx>
> <snip, original patch here: [1] >

A long long time ago I wrote a docker container image based on
dm-snapshot that is vaguely similar to this one. It is still
available, but nobody really uses it. It has several weaknesses. First
of all the container image is an actual filesystem, so you need to
pre-allocate a fixed max size for images at construction time.
Secondly, all the lvm volume changes and mounts during runtime caused
weird behaviour (especially at scale) that was painful to manage (just
search the docker issue tracker for devmapper backend). In the end
everyone moved to a filesystem based implementation (overlayfs based).

> I appreciate that this work is being done with an eye toward
> containerd "community" and standardization but based on my limited
> research it appears that this format of OCI image storage/use is only
> used by Alibaba? (but I could be wrong...)
> But you'd do well to explain why the userspace solution isn't
> acceptable. Are there security issues that moving the implementation
> to kernel addresses?
> I also have doubts that this solution is _actually_ more performant
> than a proper filesystem based solution that allows page cache sharing
> of container image data across multiple containers.

This solution doesn't even allow page cache sharing between shared
layers (like current containers do), much less between independent

> There is an active discussion about, and active development effort
> for, using overlayfs + erofs for container images. I'm reluctant to
> merge this DM based container image approach without wider consensus
> from other container stakeholders.
> But short of reaching wider consensus on the need for these DM
> targets: there is nothing preventing you from carrying these changes
> in your alibaba kernel.

Erofs already has some block-level support for container images (with
nydus), and composefs works with current in-kernel EROFS+overlayfs.
And this new approach doesn't help for the IMHO current weak spot we
have, which is unprivileged container images.

Also, while OCI artifacts can be used to store any kind of image
formats (or any other kind of file) I think for an actual standardized
new image format it would be better to work with the OCI org to come
up with a OCI v2 standard image format.

But, I don't really speak for the block layer developers, so take my
opinions with a pinch of salt.

Alexander Larsson Red Hat, Inc
alexl@xxxxxxxxxx alexander.larsson@xxxxxxxxx