[CentOS] Cannot open: No space left on device

did you (or someone else with root access) possibly delete a very large
file in /var that may still have been in use?    it's very annoying but
if you do a rm on a large file under /var that is still open by some
process for writing, it won't actually clear the space.   you can
overcome that by just truncating the file instead of doing an rm (e.g.
either > /var/log/bigfile or cp /dev/null /var/log/bigfile).   the only
way I know to fix the problem once you're having it is to force the
process to close/reopen the file... either by killing & restarting or
getting the process to do it if it's got that designed in.   in practice
I've often found it much easier to reboot a machine to fix such a
problem condition.

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public gpg key id: AE60F64C