My latest article for LWN explains (!) soft updates. The “(!)” is because soft updates are notoriously difficult to understand. If you go to a file systems conference and get people drunk, they will eventually confide to you that they don’t really understand soft updates either.
This is a free link; if you like the article, please consider subscribing to LWN. You’ll still need an account if you want to make snide comments on the article. :)