The I/O Stall Time alarm becomes active when the average I/O wait time for any database file exceeds a threshold. This value is taken over a specific number of background collections.
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Sustained high I/O wait times are a good indicator that you have a disk subsystem bottleneck and that I/O device service times will be degraded.

When the alarm is raised look at the SQL Server | Wait Events drilldown | IO Category to see what is contributing to I/O consumption. If SQL Server is contributing to I/O consumption look at the following:

  • The SQL Server | SQL Activity drilldown | I/O by File page to see which files are waiting on I/O operations. High rates of wait on log files may indicate that the device on which the log file resides is not able to keep up.
  • The SQL Server | SQL Activity drilldown | Sessions page to see which SQL Server sessions are generating high amounts of disk activity. Select a session; the Statement tab for a selected session provides for further analysis.
  • The SQL Server | Wait Statistics drilldown. Looking at the top statements by Avg Logical Reads/Writes shows SQL that may be doing large amounts of logical I/O. This logical I/O may result in unnecessary physical I/O.