Well... we are two weeks further and I can't say there's good progress.
I have used all settings exactly like Sèverin told me. Every now and then I got one backup. Since I saw the process never runs long, I just decided to start it every half hour now. This is what I get:
I have 219 backups. Sometimes it just doesn't do something for 4 days, than suddenly I get 4 more backups...
Just to be clear: I'm not running this from Moodle, but from a seperate cron outside Moodle that directly calls for the backup script.
Don't think I will ever get this working properly... and I have no idea what Moodle is doing.
And just to be clear: we have a huge server nog, 16 processors, lots of gigs and diskspace etc. Backup process gets 2048M (see above post of Sèverin)