Ext3COW (Wayback Machine link because the site’s currently being slashdotted) looks really interesting. It basically provides the same sort of “storage VCR” functionality that we were working on at Revivio. Of course, it’s filesystem level rather than block level, and it’s Linux-specific (probably even Linux-kernel-version-specific), and it’s not highly available, but it’s still cool. I wish I could read the paper so I could see how they addressed the problem of managing all of the metadata efficiently. Maybe tomorrow.