At work we're exploring ways of scaling our database layer, ideally achieving horizontal scalability in both read and write throughput. A few years ago horizontal write scalability could only mean you either explicitly shard or adopt one of the distributed NoSQL key-value stores.

I've sat on the sidelines though as "distributed SQL" became a new category- first with Cloud Spanner, then with CockroachDb, TiDB, MariaDb XPand, etc. They promise near-linear horizontally scalability, atomic transactions across instances (required for us), real online schema changes, and a compliant SQL interface.

It seems almost too good to be true if you're already tied to relational databases, which we are. I never hear about real companies with massive scaling needs using them though. Are they mature enough to be trusted? Does anyone have real-world data about performance or reliability? The literature is sparse.