X) Number of cgroups. We were getting slow performance, apparently related to slow IO, but nothing stood out as being the culprit. Turns out, since vsftpd was creating cgroups and not removing them, the pseudo-filesystem /sys/fs/cgroup had myriads of subdirectories (each representing a cgroup), and whenever something wanted to create a new cgroup or access the list of cgroups, this counted as listing that pseudo-directory, which counted as IO.

Fixed by using the undocumented option isolate_network=NO in vsftpd.conf.