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A process that's stuck on an NFS operation is unkillable, even by a SIGKILL from the root user. So any shutdown/reboot procedure that has a "kill all processes" step will be unable to complete (unless it has a timeout).

It sounds like systemctl --force --force reboot will just invoke the `reboot` syscall directly and thus avoid this issue.

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Yeah, that happens when a process is in D state (uninterruptible sleep). This happens for example if the process is waiting for io from a block device that has gone forever, like an nfs mount with a broken connection, but i also remember this happening in a company where somebody had the unfortunate idea of “retrofitting” (I’m using kind words here…) zfs to run on some rhel6 hosts, and it would hang very often (of course… but hey, we get snapshots without paying for a netapp).

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