I have a fast free isp server that expires posts after 30 days, plus a good paid server.
Running the free server as primary and paid server as backup works. However, when fetching older articles the primary server takes time to return "not found". This slows backup downloads to about 1/3 speed, regardless of the number of connections (and needlessly hammers the primary).
So, I leave the paid server as primary unless I'm fetching something recent, which means I can't queue up a mixed batch of big recent files and some older ones without using up my paid quota. I therefore switch back and forth between primary servers a lot.
It would be very helpful to have an optional age cutoff for each server, to be used only if there are other active primaries or backups (and when the age of the file is already known).
Thanks for providing an excellent tool, and your frequent updates!