Will enabling the debug shell affect the systemd shutdown sequence?

Question

I'm troubleshooting an issue where CentOS stops at "Reached target Shutdown" when I issue a reboot command. In other words the system does not reboot but remains as is after printing the mentioned message.

I've enabled the debug shell using: systemctl enable debug-shell.service

On reaching the Shutdown target I see that I am still able to access the filesystem using ls, pwd etc. from the debug shell implying that the filesystem was not unmounted. Does this mean the debug shell service has somehow prevented filesystems from unmounting? If so, how do I run a debug shell without circumventing filesystem unmount?

Answer 1

In short, yes, having the debug shell around will affect the shutdown process, perhaps preventing systemd from unmounting filesystems in the normal order. But that shouldn't really prevent systemd from eventually being able to unmount them before rebooting.

systemd tries really hard to unmount everything while stopping units normally, but then once everything is down that should be down, it will go on a killing spree, terminating all processes that are still up (and that includes the debug shell) and then it will try to unmount everything that is still mounted.

If you think the debug shell might be causing issues that are interfering with your troubleshooting of your other issue, see Diagnosing Shutdown Problems for alternative techniques that might help you debug the problem, without some of the side effects of the debug shell.