Have enabled archiving topics. Adjusted. Saved. My processes are started via CRON. But archiving doesn 't start. What did I forget to do?

It takes some time - how long have you given it to archive?

After 2 days, the topics were already archived. I looked at the database, messages in the usual unarchived form in ibf_forums_archive_posts, but in a different table. What is the point of archiving then?

Archiving moves posts out of the main posts table into an archived posts table. This can make the forums_posts table smaller, which will improve performance for any sort of action that hits the posts table.

i.e. instead of 10,000,000 posts in the forums_posts table, if you end up with 4,000,000 and 6,000,000 in a backup table, then any action against the main posts table will undoubtedly perform faster.

The forums_archive_posts table can even be stored in a completely separate database if appropriate for your environment.

 

 

can you 'unarchive' posts/topics later on after archiving them?

Yes

If you delete a section on your forums where those posts were and unarchive them what happens to those topics/posts?

I ask because I think that's what I've done

I believe if you delete the section it will delete/move those posts with it during the move/delete action. @bfarber ?

Yes, if you delete a forum and choose to delete the contents within it, that will include archived posts. If you choose to move topics when you delete the forum, it will include archived posts.