Recently I was troubleshooting slow bootup times with an LXC container I’d created. The container previously was running well, but after an update seemed to be running slowly. I restored it from a backup but still experienced slowness so, I investigated. The first really helpful step I found to take was to log out the bootup sequence of the container.
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lxc-start -n <container-id> -F --logfile=lxc.log --logpriority=debug
This command follows the boot process and logs out every step. Doing so I was able to find the hung systemd service
[ OK ] Started containerd container runtime.
[FAILED] Failed to start Wait for network to be configured by ifupdown.
See 'systemctl status ifupdown-wait-online.service' for details.
[ OK ] Reached target Network is Online.
Starting Docker Application Container Engine...
From here it looked like an issue with some service waiting for network-online.target so I checked which services were dependent
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systemctl show -p WantedBy network-online.target
I also ran the following to determine which exact service was causing the hangup.
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systemd-analyze blame
Between the two, I found a service I had created for a specific network adapter that I had since deleted. This was causing my issue, once I unregister the service and it booted normally. An easy solution once I had visibility into an otherwise vexing problem.