To expand on this question, I'm wondering how useful daily backups even are for a site like Facebook. I mean, of _course_ you need them, but also, something about reverting all of FB to a state 24 hours ago seems disastrous even if it works. I can't imagine that it's an acceptable thing in anything but an absolute emergency. Imagine every single facebook user got rewound in time to the previous day, every message sent over the past day was lost, etc.

It was a lifetime ago I ever did DB administration (postgres in my case), but the write-ahead-logs being replicated out independently was extremely important for point-in-time recovery, such that you could always take the latest backup, zip forward through the WAL, and recover to any arbitrary point in time you want, so long as the WALs were available. I wonder how much something like this would have been done at FB scale.