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.
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
The I/O category on the SQL Server | Wait Events Drilldown to see what is contributing to I/O consumption. If SQL Server is contributing to I/O consumption look at the following:
The Sessions page on the SQL Server | SQL Activity Drilldown to see which SQL Server sessions are generating high amounts of disk activity. The SQL and the query plan are available for further analysis by clicking a row in the grid.
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.