today we have exceprienced really major difficulties with our proxy backend servers. Everything started after enabling auth cache:
auth_cache_size = 100M
auth_cache_verify_password_with_worker = yes
auth_cache_ttl = 1 hour
auth_cache_negative_ttl = 1 hour

Soon after we start receiving lots of calls from our customers about 'missing all e-mail messages'. This was in errors logs on master server:

Apr 16 14:37:10 server00 dovecot: imap(<censored_email_address>): Error: autoexpunge: Couldn't create dovecot.autoexpunge.lock lock: file_create_locked(/var/mail/vhosts/<censored_domain>/<censored_name>/home/dovecot.autoexpunge.lock) failed: safe_mkstemp(/var/mail/vhosts/<censored_domain>/<censored_name>/home/dovecot.autoexpunge.lock) failed: No such file or directory

Looks like that all users, who are placed on proxy backends, were logged correctly BUT not proxied to the right server so they saw empty mailboxes (Dovecot also created some directories on master server). What is worse, their email software deleted all local emails so they are now downloading everything from scratch (100Gs of mails so it will take some time).

After auth cache was disabled, things started to go to normal EXCEPT users were reporting they don't see all folders - which was true, only few on them were visible (INBOX, trash, sent etc.). We tried everything to make them visible again, for example deleting indexes, but nothing helped. Finally, removing option ITERINDEX from mail_location helped.

Can anyone explain what happened? Thank you


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