After finally making it through an upgrade of 5.0.20 to 6.0.4.
The new monitor started alerting us that our backup volume was getting a bit low (93%).
Since we had it (the entire volume) backed up elsewhere, I surmised it was because of all the 3 or 4 "Full" backups I had done prior to the upgrade.
So I went in as Zimbra, and issued zmbackup -del 25d and saw the available space increase slightly, as it deleted a few old Incrementals.
My plan was to decrease 25d until I got below the magic 85%.
However it stopped freeing up space after about 20d.
So, since I had a full volume backup anyway, I decided to clear all the backups, still had 90% used....
I then investigated the /backup/sessions/ folder and found a bunch of subfolders with "TO_DELETE-" prepended to them.
These were "current" backups too.....
Nothing in the docs for zmbackup mentions that there is a delay in purging the entries so what gives?
In the end I used rm -rf to trash everything in the /backup directory and started again.