A regional disk is a logical disk that is synchronously replicated at the block level across exactly two zones within the same region [1]. Since the disks are always identical, with no replication lag, the HA control plane can seamless fail over the whole database to a new master that plugs into the same disk. It's all in the article.

Regional disks aren't publicly available yet, but they are in alpha [2]. Like normal persistent disks, everything is backed by Google's internal Colossus system [3].

[1] https://cloudplatform.googleblog.com/2017/11/Cloud-SQL-for-P...

[2] https://cloud.google.com/sdk/gcloud/reference/alpha/compute/...

[3] https://cloud.google.com/files/storage_architecture_and_chal...