What yuliyp wrote is basically it. Although the individual shards weren't really small, even by modern standards.
> Considering all the transactions in flight, and everything?
If I remember, we used --flush-logs --master-data=2 --single-transaction, giving it a consistent point-in-time dump of the schemas, with a recorded starting point for binlog replays, enabling point-in-time and up-to-the-minute restores. Nowadays you have GTIDs so these flags are obsolete (except --single-transaction).
--single-transaction does put extra load on the database—I think it was undo logs? it's been a minute—which caused some hair-pulling, and I believe they eventually moved to xtrabackup, before RocksDB. But binary backups were too space-hungry when I was there, so we made it work.
Another unexpected advantage of mysqldump vs. xtrabackup, besides size, was when a disk error caused silent data corruption on the underlying file system. Mysqldump often read from InnoDB's buffer cache, which still had the good data in it. Or if the bad block was paged back in, it wouldn't have a valid structure and mysqld would panic, so we knew we had to restore.
> And did you ever Test disaster recovery with that setup?
Yes! I wrote the first version of ORC. This blog post is from long after I left, but it's a good summary of how it worked: https://engineering.fb.com/2016/10/28/data-infrastructure/co...
It wasn't the best code (sorry Divij)—the main thing I'm proud of was the recursive acronym and the silly Warcraft theme. But it did the job.
Two things I remember about developing ORC:
1) The first version was an utter disaster. I was just learning Python, and I hit the global interpreter lock super hard, type-error crashes everywhere, etc. I ended up abandoning the project and restarting it a few months later, which became ORC. In the interim I did a few other Python projects and got somewhat better.
2) Another blocker the first version had was that the clients updated their status by doing SELECT...FOR UPDATE to a central table, en masse, which turns out to be really bad practice with MySQL. The database got lock-jammed, and I remember Domas Mituzas walking over to my desk demanding to know what I was doing. Hey, I never said I was a DBA! Anyway, that's why ORC ended up having the Warchief/Peon model—the Peons would push their status to the Warchief (or be polled, I forgot), so there was only a single writer to that table, requiring no hare-brained locking.