ok, I see.
No, it's not an option, we want process instance to be persited to DB.
But we want to delete old process instances...
I just found out, that the completed (or explicitely aborted) process instances do in fact get deleted, that's good!
But still there is a lot of abandoned active instances that just did not run till completion, and they are staying in the DB. What would be the the way to clean them??