system wakeup caused by write operations to /var/lib/dovecot/instances

[Tijl]

Hi, I hope someone can help me with this. 

Problem
Dovecot writes to _/var/lib/dovecot/instances_ 1 up to 3 times a day,
causing the system to wake up and disks to spin up. 

Details
content of /var/lib/dovecot/instances :
1549073010 dovecot /var/run/dovecot /etc/dovecot/dovecot.conf
--> only the timestamp is changed (to current time) when dovecot writes
to this file
--> during write the file is temporary renamed to _instances.lock_ 

dovecot running processes:
dovecot (main process) --> it's this process that writes to
/var/lib/instances
dovecot/anvil
dovecot/log
dovecot/config --> sometimes this process is not running, but doesn't
change anything

note:
no clients connected
problem also occur in the middle of the night (no human activity, client
devices powered off, all external traffic blocked)
the writings occur around every 8 hours, but vary, sometimes it's 6
hours up to 13 hours. 

Configuration
See attachment:
doveconfig-short-[platform].txt (output of _"/usr/sbin/dovecot -n"_)
doveconfig-long-[platform].txt (output of _"/usr/bin/doveconf"_)
--> _3-docker_ config is the most simple setup of the 3

Environment
Tested on 3 platforms
Platform 1:
Intel Atom C2538 (Synology DiskStation 415+)
Linux version 3.10.105
Dovecot version 2.2.19 (Synology MailPlus Server) 

Platform 2:
ARMv7 rev 1 (Synology DiskStation 215j)
Linux version 3.2.40
Dovecot version 2.2.19 (Synology Mail Server) 

Platform 3:
Intel Atom C2538 (Docker container)
Linux version 3.10.105
Dovecot version 2.3.2.1 (0719df592) 

How can dovecot be run without writing to /var/lib/dovecot/instances
everyday? Is there a configuration setting for this? 

Thanks already!
Tijl
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