This is a great solution for developers with any legacy code. It's easy to fire up a new project and begin with a fun, new NoSQL storage engine when you're starting with a blank sheet of paper, but it's much harder to take running code and convert it. I was able, for instance, to get Drupal up and running in less than five minutes because Drupal relies upon MySQL to store all of the data. I didn't need to rewrite the Drupal code to work with some new object or document store. There was no weird glue code or translation architecture. I just started up a MySQL database and pointed the Drupal code at the URL.
The separate MySQL option came around because the engineers at Rackspace listened to customer complaints that the performance of databases often wasn't as good as it could be. The virtualization layer used in clouds like these added delays in writing and reading from the I/O, a factor for operations like running a database. The device driver for your virtual machine won't write directly to the disk, but will shove the data into shared memory and wait for the underlying machine OS to actually write it to storage. That may be an acceptable price to pay for some applications, but not for code that lives to store data to a hard disk.
Rackspace's solution is to eliminate some of the hardware and operating system layers, which the company calls "container-based virtualization." You can't log into your MySQL database server and configure the underlying OS. You get only a URL and a MySQL user password; all of your interaction happens as a MySQL user, not as a regular Unix user.
Protecting your cloud dataRackspace has added extra redundancy out of sight. The version of MySQL isn't running on any old machine, but writing to a SAN with RAID hardware. Rackspace then enables further protection by copying the data to other machines on the network. All of this replication happens at the hardware or network level, not with MySQL. Rackspace doesn't currently use the MySQL replication code, although it promises to offer that in the future. The company also promises to offer yet another layer of protection, an automated backup tool for taking snapshots of your database.