> They don't scale to Google or Facebook operational sizes.
You may or may not know this, but the primary datastore used at both Google and FB is MySQL. Sure, they use replication and sharding, but I would strongly argue that MySQL with sharding scales better than some multi master NoSQL thing like Cassandra.
Related, you should check out https://github.com/vitessio/vitess if you haven't seen it. It's what Youtube and others use for their primary data store in production.
> You may or may not know this
I didn't, but they do, surprising!
https://www.facebook.com/notes/facebook-engineering/tao-the-...
> but the primary datastore used at both Google and FB is MySQL
I'm almost certain this isn't true, BigTable and Spanner are much more widely used at Google because.... well, MySql doesn't really scale.
They don't scale to Google or Facebook operational sizes. Once you get to a billion customers or so the ol' RDBMS tends to struggle. Because everyone wants to be Google they imagine they have Google's problems. I've been in a meeting where the client was talking about their severe scaling issues for their "big data" which could only possibly be resolved by state of the art cloud solutions. I pressed them on the numbers - they had 400GB. You can buy an iPhone with 512GB.
Oracle's Exadata X8-2 is ~1PB per full rack.