`/var/lib/prometheus/postfix-exporter/mail.log` is growing indefintely

[jbartak]

Hi, we started using this exporter for monitoring our mail servers.
It runs in very default config. We've noticed it is writing into /var/lib/prometheus/postfix-exporter/mail.log and it size if growing indefinitely.

Right now:

|Debian-11|[root@qla-prod-ue1-la-smtp-01-1 bin]$ ls -lah /var/lib/prometheus/postfix-exporter/mail.log 
-rw-r----- 1 prometheus prometheus 6.1G Jul  3 18:41 /var/lib/prometheus/postfix-exporter/mail.log

How we should address this? Logrotate?
And why it needs to duplicate config mail.log when it is available in /var/log/mail.log?

[dswarbrick]

You don't provide any concrete information about your environment, e.g. Linux distro, whether you are using a distro package, vanilla upstream binary, or buildt the exporter yourself from source.

However, I'm going to assume that the "Debian-11" hostname suggests that you're running this on Debian 11 (bullseye). I maintain the prometheus-postfix-exporter package in Debian, so I am aware that some older versions of the package did not truncate or rotate the exporter-specific mail log in /var/lib/prometheus. The Debian prometheus-postfix-exporter never shipped a logrotate snippet, because it relied on the exporter truncating the file after reading it - which it did, prior to v2.0.

This issue was fixed in the Debian package by this commit. The commit message clarifies further:

postfix_exporter (up to and including version 0.1.2) truncated the mail log file after reading it, and thus required its own extra log, so as not to interfere with the default /var/log/mail.log.

However since version 0.2.0, postfix_exporter no longer truncates the log file, and instead uses a tailer which seeks to the end of the log upon start, and thus only reads new log entries as they are added. This means that, without a logrotate config snippet, the additional private log file would have continually grown.

As there is no longer any need for an extra mail log file, drop the config / patches associated with it, and simply use the system default /var/log/mail.log.

The fix first appeared in version 0.3.0-2 of the prometheus-postfix-exporter package, meaning that Debian 12 (bookworm) included the fix.