This command quickly pointed to ~/mdbox/mailboxes/INBOX/dbox-Mails/dovecot.index.tmp That file was written excessively.
Was it one user's dovecot.index.tmp or for a lot of users?
There's just one user. All mail goes to one mailbox.
This means that dovecot.index is being rewritten, which should happen only once in a while, but now it sounds like it's happening maybe for every mail delivery.
Yes, it seems to be that way.
If it's still happening, could you send me one folder's dovecot.index and dovecot.index.log files? (They don't contain anything sensitive other than maybe message flags.)
I can send the files.
This is dovecot 2.3.7.2-1~bionic
So you had been running this version already for a while, and then it just suddenly started getting slow?
Yes. Initially I threw away the whole mailbox after it got slow, but after a few days the performance started to degrade. Admittedly, it contains a lot of mails :)
I tried to reproduce this with imaptest and Dovecot that is patched to log when dovecot.index is being rewritten, but there doesn't seem to be any difference with v2.2.36, v2.3.7 or git master.
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