Back in 2003 I was a developer on a team at Microsoft that was responsible for building a new storage backend for all the MSN Messenger and Hotmail contact lists. This store had to hold data for close to 300 million user accounts, which we sharded out to a few hundred SQL server databases. Our sharding system consisted of a database with a single table with a row for each user that mapped their 128 bit guid user ID to the ID of their assigned shard. New user creation involved generating a new guid and inserting into this table. Each read operation involved a select from this table.

The database ran on a machine with enough RAM to let SQL Server do its thing and cache almost the whole table in memory. At the time I left the team in 2005, it was executing over 25,000 requests per second, with an average latency of under 3ms. Pretty sure that on modern hardware it would handle much, much more.