Azure SQL Database Limits

Let’s talk briefly about resource limits with Azure SQL Database. Because we have to share resources with other users, and because Microsoft doesn’t want us affecting others dramatically, they have implemented some limits.

If for some reason our database does get overeager, the operations relating to CPU, RAM and I/O will be queued up by the underlying resource manager, which will allow those operations to continue once the resources are free. Yes, that will slow down our system.

Resources other than CPU, Memory, Log I/O, and Data I/O, as noted by the documentation, will be denied, and clients will see an error message.

Microsoft uses something called DTUs (Database Throughput Units, a combination of CPU, RAM and log writes) to figure out our Azure SQL Database’s resource usage.

Check out the DTU Calculator to find out more, and see how your on-premises database compares. Next week, we will go into more detail about DTUs, and how to calculate the DTU usage of an on-premises database.

It gets really complicated, really quickly, but using the DTU Calculator before we decide to move to Azure SQL Database, and being conservative with our estimates, will make our lives easier.

The biggest an Azure SQL Database can be on the Standard Tier, is 250 GB. The biggest it can be on the Premium Tier, is 1000 GB.

Coincidentally, the SQL Server Customer Advisory Team (SQLCAT) wrote a blog post titled “Real-World Azure SQL DB: Unexpected Database Maximum Size Limit” last month, about a gotcha when upgrading between service tiers. It is recommended reading.

It helps to draw parallels between Standard Tier and Standard Edition of SQL Server, and Premium Tier and Enterprise Edition of SQL Server.

(Edit: With SQL Server 2016 Service Pack 1, you can now create Memory-Optimized objects in all editions of SQL Server.)

There is a Basic Tier for Azure SQL Database, but the resource limits are so strict that I barely consider it. Perhaps if you run a small MySQL database for a WordPress website, with a caching plugin, this may be appropriate.

Using elastic pools (the ability to share the count of resource DTUs between several databases in a pool, that shrinks and grows as your requirements change) can help with balancing costs across several databases, but it is going to seem more expensive at the outset.

The bottom line with Azure SQL Server is that the resource limits may take you by surprise if you don’t plan correctly. Spend the time to evaluate the requirements of your database, and don’t just use Azure SQL Database because it’s new and shiny. If you currently have an Enterprise licence for SQL Server, chances are you’ll spend the equivalent (or more) on a Premium Tier database.

Azure SQL Database is not a way to save money on your SQL Server licences. It is a different technology, with different resource requirements, that cater to a specific workload. Choose wisely.

If you have some thoughts about DTUs, come and talk to me on Twitter, at @bornsql.

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