Maybe you should disable quota on btrfs backend

I have a lxd 3.0.1@ubuntu18.04 running docker inside the lxd with btrfs , when quota was enabled on any lxd container in the same storage pool, btrfs will freeze while docker container create/destroy frequently .

disable quota fix this.

it seems a bug : FS access deadlock with btrfs quotas enabled

I have seen this behavior for a few years now. When we first deployed containers, we used btrfs with quotas and snapshots. After a few weeks of creating/destroying snapshots, we noticed the CPU would hit 100% during snapshot removal. This would essentially freeze all running containers since the disk I/O wait was extremely high (even with SSDs).

The fix: disable btrfs qutoas.