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Troubleshooting High I/O Usage on Azure SQL DB

Etienne Lopes troubleshoots a strange issue:

After the downsizing (to GeneralPurpose: Standard-series (Gen5), 2 vCores) occasionally there were timeouts in the application for a very specific task (the command timeout property in the application was set to 30 seconds). Other times the very same task would execute immediately, as it should always, since the underlying query was actually quite simple: a SELECT to a single, although large table (58 GB) but with a predicate that would always result in a perfect index seek to return never more than 300 rows. Furthermore each time there were timeouts, there were also momentary I/O spikes up to 100%:

Read on to learn more about what caused this problem and how Etienne was able to resolve it.