I have, finally, managed to run obnam on all files in my home directory, save music files, and DVD rips, neither of which I want to back up at this time.
It is now time to check that I can run forget on everything except the latest generation.
Did a hard-link copy of the backup store first.
Disk usage before forget: 236 GiB.
Time to run forget: much too long...
Afterrunning this for six hours, it still hasn't finished. That's not practical. I'll kill it. Some day I'll start working on speeding it up.