Azure SQL Managed Instance versus Amazon RDS for SQL Server—Which Should You Choose? (Or why Managed Instance is faster)

Microsoft, in conjunction with Principle Technologies recently produced a benchmark, comparing the performance of Azure SQL Managed Instance, and Amazon RDS SQL Server. I normally really dislike these benchmarks—it can be really hard to build proper comparisons and the services frequently don’t have perfect equivalent service tiers, making them really hard to ultimately compare their performance. In fact, when I was reading this benchmark, I saw something when comparing the two services that made my eyes light up. And then I realized it was a limitation of RDS.

I immediately saw that Azure MI had 320,000 IOPs while AWS only had 64,000. Obviously Azure is going to crush any database benchmark with that difference. And then I did a bit more research. I the visited the AWS docs.

You’ll note that while Oracle RDS does get up to 256,000 IOPs (I guess those customers have more money), SQL Server has a max number of IOPs of 64,000. Needless to say, in this benchmark comparing the price/performance ratio, Managed Instance crushes RDS before you even add in the hybrid licensing benefits that Microsoft supports for Azure SQL services.

But Wait There’s More

While Managed Instance is by no means a perfect service, there are a number of reasons why I strongly recommend against running your database on RDS. Here are the main ones:

  • You can’t migrate a TDE encrypted database using backup and restore—you have to extract a BACPAC and import into a database in the service
  • The native backup solution doesn’t support restoring a database to point in time.
  • You can’t deploy cross-region, meaning, there is no near real-time option for disaster recovery
  • There is no instant file initialization which can make some restore and file growth operations extra painful

These are the major concerns, with an additional licensing concern of not being able to use Developer edition for your workloads, means your overall costs to run an environment are going to be a lot higher of you use RDS.

When Not to Choose PaaS?

While RDS has a lot of costs associated with it, and performance is limited, there is a price/performance/data volume curve that I feel I applies to both Azure and AWS platforms. If you need high end storage performance (which means using the Business Critical service tier), on Managed Instance, and your data volume, is more than a terabyte, you have to scale your Managed Instance to 24 cores. If your volume is more than 2 terabytes, you need to scale to 32 cores, and if you need more than 8-16 TB, you will need to scale to 80 cores, which will cost close to $30,000/month. I perfectly understand why this is the cost model—the storage is stored locally on the VM itself rather than remote—so Microsoft can’t put other VMs on that piece of physical hardware.

What Should You Do for SQL Server on AWS?

If you need to run SQL Server an AWS, what should you do? The answer is to use EC2 VMs. Sure you lose the minor benefits of having your servers patched and limited benefits of the backup feature, but you have more granular control over your IO performance and overall configuration.

Tl;dr Azure SQL Managed Instance delivers a lot more throughput than Amazon RDS for SQL Server, so your workloads will run a lot faster on Azure.

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