[pve-devel] Proposal For Podman Container Support

[Thomas Lamprecht]

Hi Raven King,

I want to say thanks up-front for trying to improve on of our open
source projects and reaching out upfront for doing so, highly
appreciated.

Am 13.03.25 um 19:03 schrieb Raven King:
> This is my first time writing to this mailing list. I have never 
> contributed to proxmox but I would like to try and write a feature that 
> allows native container support (not inside an LXC or VM).

FWIW, LXC definitively are "native containers", it's less confusing
to use application containers (like OCI conform ones) and system
containers (running a full distro), as LXC can be used for both,
i.e. docker used LXC for a while for isolating application CTs.

> My goal would be that you could manage those containers much like 
> LXC/VMs with similar UI behavior (resource usage views, easy access to 
> container console, and resource sharing).
> Its a large undertaking, and I would probably want to get a little 
> experience with the proxmox codebase first.

I think we should take a step back and not focus on integrating podman
too much, but rather about adding support directly in our existing
container toolkit. Actually I'm pondering about adding support for the
OCI runtime spec [0] and maybe also the OCI image spec [1] over the
last years and had some light talks with developers about that, so I
think you and I agree on the end goal already, but not on the road or
method to get there.

[0]: https://github.com/opencontainers/runtime-spec
[1]: https://github.com/opencontainers/image-spec

> 
> *Why do this?*
>      1. It is parroted by users frequently. Just look up "run docker in 
> proxmox" and you will see dozens.
>      2. It would add a major use case to proxmox.
>      3. For me personally, it removes a major pain point of using 
> proxmox, which is setting up an LXC to then share resources with to then 
> setup a docker image to then share resources with.
>          Or using docker directly and tearing my hair out as it 
> magically breaks all my proxmox network config.

The why's are all fine, the reason it does not exist yet is not because
we saw no reason, but rather because there are good workarounds that
can be used and correctly implementing this into our container runtime
while ensuring as much as possible is shared with our existing
implementation.

> 
> *Why Podman?*
>      1. Easy enough to use.
>      2. Packaging. The support in debian is straightforward and won't 
> confuse anyone. This means the project won't have to maintain podman 
> itself in any way.
>      3. Security. Podman needs limited privileges to operate compared to 
> docker. This makes it easier to mesh with things such as user accounts.
>      4. Interop. It easily goes to/from kubernetes, which can help in 
> some enterprise use cases. Also doesn't interact in ways that break 
> existing pve config mechanisms.
> 
> *What does podman offer an LXC doesn't?*
>      1. Easy deployment, you can just pull images that someone prebuilt 
> for a purpose, including most docker images.
>      2. Directly sharing a host directory (not a whole drive) such as 
> single zfs datastore. While achievable in LXC, you have to do a bunch of 
> user mapping and the setup is rather involved.

You're mixing things here, LXC is a lower-level technology, it simply
does not care about image management and certainly does not limit PVE
on sharing CT and/or host directories at all. LXC rather provide
building blocks that can definitively be used to support these things.

> *What drawbacks have I considered?
> *1. Using privileged ports in a podman container is a little tricky 
> without root. Proxmox mostly runs as root though, so this is really only 
> a problem for secondary users.
> 2. I will take a lot of work to ensure the networking works in a way 
> consistent with other networking in proxmox.
> 3. Increase support burden as users who aren't entirely familiar with 
> docker/podman containers ask questions that could be answered through 
> research.
> 4. Some services people might want to run, such as nginx proxy manager, 
> are gonna be very hard to use in this way due to number 1.

IMO above are not that significant but rather minor/mid-level technical
hurdles that can be overcome, the biggest drawbacks of using podman
directly are IMO rather:

- two CT stack we need to support, ours and a third-party one
- dependency on third party developers and a programming language (Go)
  we do not use at all in any of our projects.
- while great software, it does not align _that_ well with Proxmox VE's
  ways of things, thus a clean and good integration that feels native
  to PVE, and not just tacked on, is IMO rather hard to do.

That all means quite some high permanent maintenance cost, which is what
would have to bear, so I'm rather opposed to it, at least without some
concrete plan of someone having intermediate+ experience with not just
working with PVE but also internal development, as otherwise it's IMO
too hard to ensure above concerns are unfounded or not relevant.
In general, I'd highly favor a native implementation that we have control
over and can neatly integrate into Proxmox VE and all its features (SDN,
backup, HA that gets extended by orchestration ...), and given that
there are specifications for what we want to support here, that should
be (hopefully) doable without extreme effort, and ideally with not much
more effort than integrating complete solutions like podman, at least
if one also minds the long term maintenance cost.

> I am writing to the mailing list before even beginning on this endeavor 
> to get several questions answered:
> 
> 1. Do y'all have any general tips and pointers about navigating and 
> working with the proxmox codebase?

There is some basic info here:
https://pve.proxmox.com/wiki/Developer_Documentation

Definitively does not cover all the things though, but basic patch
handling should be described.

> 2. Where is a good list to grab bugs to get familiar with proxmox 
> structure and what functionality is where? I have some hardware, but I 
> am not capable of testing stuff like multi-gpu setups.
>      I see 
> https://bugzilla.proxmox.com/describecomponents.cgi?product=pve but 
> there is a lot of components to proxmox. I have a hard time picking a 
> spot to start.

We try to add a "start-contributing" tag to some issue requests, i.e.:
https://bugzilla.proxmox.com/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=UNDECIDED&bug_status=REOPENED&bug_status=MORE%20INFO%20NEEDED&bug_status=POSTPONED&list_id=5791&longdesc=\btag%3A\s*start-contributing&longdesc_type=regexp&query_format=advanced&resolution=---

This can be filtered for the "container" component, albeit I did not
ensured the three issues that come up are valid:

https://bugzilla.proxmox.com/buglist.cgi?bug_status=UNCONFIRMED&bug_status=NEW&bug_status=ASSIGNED&bug_status=REOPENED&bug_status=MORE%20INFO%20NEEDED&bug_status=UNDECIDED&bug_status=POSTPONED&component=Container%2FLXC&list_id=50251&longdesc=%5Cbtag%3A%5Cs%2Astart-contributing&longdesc_type=regexp&order=Importance&product=pve&query_format=advanced&resolution=---

> 3. Are there any major drawbacks to container support that need 
> consideration?

Just to ensure we speak of the same: Containers are already supported
in general, albeit our runtime that wraps LXC and co is targeting
system containers, not application ones.

> 4. Are there specific drawbacks to podman that need consideration?

See above.

> 5. Anything else I am overlooking with this idea?

The way I'd get started is evaluating the OCI specs, pve-container and
potentially also existing runtimes that implement the OCI specs.
For then implementing the spec and integrating that into pve-container
we would definitively open, even lightly encouraging, doing so in rust,
maybe at least the lower level building blocks for understanding/parsing
formats as defined in the OCI specs. Then use perlmod [2] to integrate
that rust modules into the existing pve-container Perl based code.
You could also just stay in perl, that would be fine for us, but in
general we try to use rust if possible for new (bigger) features and
also find that it provides us with a lot of guarantees and modern
language features to make one lives easier in the long term.

[2]: https://git.proxmox.com/?p=perlmod.git;a=blob;f=README.md;hb=HEAD

That's naturally a lot to ask for a new contributor, but if it was
easy it would have been already done, and we simply want to favor
native and well integrated solutions to avoid external dependencies,
of which drawbacks we had to deal with too much in the past, so we're
pretty much set on that part.

FWIW, I directly CC'd one developer I talked lightly over adding OCI
support to PVE, maybe he got some time to think over this and
spearhead the initial effort.
If you're still interested into tackling this we naturally will try
to help you on any specific question, I still appreciate you wanting
to move this forward, but I also wanted to manage expectations, as
this might be quite the task, especially for one not accustomed to
our project and its code basis.

best regards
Thomas