Amazon RDS for SQL Server has some ridiculous storage constraints

When you think about cloud computing, you think scalability, flexibility, etc. Right? Not really when you look at the Amazon Relational Database Service (RDS) for SQL Server, at least the storage part of that service. RDS is Amazon’s database PaaS (Platform as a Service) offering for relational databases, and it does a lot of things right: New instances running with a few clicks, backups fully-automated with restore to point-in-time functionality, good monitoring tools, etc. However when it comes to the storage part of the SQL Server part of RDS, there are some serious constraints:

  • You can not change the allocated data storage after creating the SQL Server instance. That’s right, you have to allocate, and pay for, the maximum storage that you will ever use in the future. Not only is this impossible to predict, but it means that you pay for a lot of unused storage, something “the cloud” was supposed to prevent.
  • When selecting guaranteed IO performance (provisioned IO per second, or IOPS) additional constraints are present:
    • The storage must be an exact multiple of 100GB. Storage costs $0,138 per GB/month.
    • The selected IOPS must be exactly 10 times the storage in GB (so with 200GB storage you must select 2000 IOPS). IOPS costs $0,11 per IOPS/month.

Now, for any serious databases use, you need reliable performance, which means that you almost always have to select the provisioned IOPS option. So if I create a database instance with 1 TB storage (the maximum, which I have to select because later expansion is not possible), I have to select the 10,000 IOPS option, and I am paying $138 per month for the storage and $1100 per month for the IOPS. In addition to that, of course, you also pay for the instance itself.

Strangely enough, the storage constraints when using Oracle and MySQL are much less strict: You can scale the storage up at any time, without downtime, and when selecting provisioned IOPS the storage can be any value between 100GB and 3TB, and the IOPS/storage factor can be any value between 3 and 10.

So what is going on here? Do Windows Server and/or SQL Server have such inherent constraints that the same storage flexibility as for Oracle/MySQL can never be achieved? Or has the Amazon RDS team just not yet had the time to fully come to grips with the Microsoft systems?

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