As we’re preparing support for full CGroupV2 on LXD, we tried installing the lxd snap on such a system.
This resulted in:
root@djanet:~# snap install lxd
error: cannot perform the following tasks:
- Run install hook of "lxd" snap if present (run hook "install": cannot open freezer cgroup (/sys/fs/cgroup/freezer): No such file or directory)
I then tried to install the simple hello-world snap, which resulted in:
root@djanet:~# snap install hello-world
hello-world 6.3 from Canonical✓ installed
root@djanet:~# hello-world
cannot open freezer cgroup (/sys/fs/cgroup/freezer): No such file or directory
So it looks like snapd is very much assuming that the freezer cgroup will always be present and will always be mounted at /sys/fs/cgroup/freezer. This sadly isn’t true with a CGroupV2 system, on which the freezer cgroup simply doesn’t exist at all.
At this point in time, there are no equivalent to the freezer cgroup controller, it wasn’t ported to V2 due to it being considered of poor quality and reliability in V1. I’m hoping this will change as its feature is genuinely useful and something we use too, however it seems likely that Linux distributions may begin the V2 switch prior to this happening.
In order to reproduce the behavior above, boot your system with this added to the boot command line:
systemd.unified_cgroup_hierarchy=true