Some backstory on this: we've been working on a storage primitive to power incremental docker builds for Blacksmith's GitHub Actions runners. We realized early on that this was going to be a useful capability on its own since we notice a ton of our users with massive cache restore/save times in their workflows.
Fun fact, we decided to call it "sticky disks" as a result of this old HN comment thread: https://news.ycombinator.com/item?id=39957753