Azure SQL Managed Instance Extreme Storage Latency

Kendra Little has another caveat emptor message:

What are your stories of unbelievably bad performance from cloud vendors? I’ll go first. For years, Azure SQL Managed Instance’s General Purpose Tier has documented “approximate” storage latency as being “5-10 ms.” This week they added a footnote: “This is an average range. Although the vast majority of IO request durations will fall under the top of the range, outliers which exceed the range are possible.”

How approximate is that 5-10 milliseconds, you might wonder? If you use Azure SQL Managed Instance these days, you will regularly find messages in your SQL Server Error log indicating that all data and log files have experienced latency of up to 60 seconds. At least, 60 seconds is the maximum I’ve observed personally, looking in the logs of several customers’ Managed Instances. Could it be worse? Microsoft hasn’t documented a ceiling. My testing shows that this latency occurs randomly to your workload and is not related to your resource usage: using less IO will not make the errors less likely. You have no way to avoid these storage failures (I don’t see how 15-60 second latency is not a failure), and they can occur anytime.

This is a major strike against SQL Managed Instance General Purpose. Considering the cost of the product, you could buy a new server with direct-attached NVMe storage, have it paid off after one year, have better performance, and get to depreciate the entire expense over a 3-5 year window (something you cannot do with the hardware side of cloud services–you can only depreciate the cost of licensing, assuming you have a 3-year reservation).