If you stop dovecot does the process in D state stops too or do they
block and you need a reboot ?
those state D process have plagued me since 3.2 but for now 3.2.17 do
not triggered them (touch wood).
What cgroup limiting and dlimit do you use on your config ? Did you
enabled any of them even if you do not use them ?
regards,
Ghislain.