I got a bit further… It seems that the daily cleanup of the Syslog-entries is broken for some reason. The entries do not get removed by the ./daily.sh script.
I just fired the command lnms maintenance:cleanup-syslog manually and it’s already running for ages. I will update this ticket when it’s finished.
EDIT: It just finished after nearly one hour runtime! 
Cleared syslog entries older than 7 days (33198601 rows)
Checking daily.sh I can not find any reference to the syslog-cleanup. Was it removed accidentally or was it never added?
And does anything speak against creating a PR containing the missing line for also cleaning syslog-entries by the daily job? If not, I’d do so.
Also, after running a mysqlcheck -o librenms the size on disk further went from now 21GB to 4.1GB! Should we also have a scheduled optimize in a cron-job? Or at least add some “best practice” notes so that the admins take care of this themselves?
cleanup)
# Cleanups
options=("refresh_alert_rules"
"refresh_device_groups"
"recalculate_device_dependencies"
"eventlog"
"authlog"
"callback"
"purgeusers"
"bill_data"
"alert_log"
"rrd_purge"
"ports_fdb"
"ports_nac"
"route"
"ports_purge")
call_daily_php "${options[@]}"
;;