You can't compare that to Spanner. RDS is a product that helps you manage/scale something like MySQL for you. It would compare to CloudSQL in the Google portfol

You can't compare that to Spanner. RDS is a product that helps you manage/scale something like MySQL for you. It would compare to CloudSQL in the Google portfolio. However, they have their own database engine called Aurora, which is what you actually get when you select MySQL or Postgres for RDS and provides much better throughput and replication behaviour. You can also only have up to 15 Aurora replicas whereas Spanner goes far far beyond that.