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Our web application is suddenly experiencing intermittently slow performance (slow page loads, slow ajax calls going from <1 sec to over 20 secs). The only clue is when the application slows, we see ASYNC_NETWORK_IO waits being reported by the MS SQL server. These errors, and the slowdown occurs under both heavy and light server loading (as determined by the number of connections).

This application has been in place for several months without problems, the only recent changes Azure-wise being the addition of auto-scaling and SQL server fallback a few months ago. The application itself is under routine development and deployments.

We have looked at the code to see if any recent coding changes could have affected the performance from an application --> SQL standpoint, but can't find anything (all database calls go through a NetTiers DAL).

Summary: MVC 4 application Azure deployment, multi-tenant with autoscale MS SQL Server, with fallback Azure Storage NetTiers DAL.

Any ideas on the intermittent performance issue would be appreciated.

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ASYNC_NETWORK_IO means one of two things.

  1. Your code isn't processing the data from the application fast enough. You've got large rowsets, or row by row processing and the application server simply can't keep up for some reason.
  2. There's a network problem.

As you are hosted in Azure odds are it's #1 and that the problem didn't show up before because the recordsets just weren't big enough to cause the problem. It is possible that the network between the app servers/web servers and the database server is simply running at capacity which would require a call to support to verify.

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