Performance Tuning with SQL Server Dynamic Management Views

Viewing pending I/O requests
Chapter 6: Physical Disk Statistics and Utilization
Whereas the data in the the sys.dm_io_virtual_file_stats DMF is cumulative, the
sys.dm_io_pending_io_requests DMV returns a row for each currently pending I/O
request, so it is interesting from the standpoint of seeing what is happening right now,
at the file level. This is a useful tool for analyzing disk I/O at a more granular level than is
allowed by tools such as Profiler, which will summarize I/O activity at the drive level.
Each time you query this DMV, it's likely that a different set of processes will be pending,
so the returned data will vary each time. However, if you frequently see what seems like a
high number of pending I/Os, this is evidence of some level of I/O bottleneck. Obviously,
"high" is a matter of interpretation, but more than two or three could indicate an issue.
This DMV returns several columns that are occasionally useful, including:
• io_completion_request_address – address of the I/O request
• io_type – type of I/O, for example 'disk'
• scheduler_address – relates to sys.dm_os_schedulers to get information
about the scheduler coordinating this I/O request
• io_handle – handle for the file that is the target of the I/O request; relates to
sys.dm_id_virtual_file_stats.file_handle to get the file information.
However, the most interesting columns returned by this view, and the ones we'll use in
our script, are:
• io_pending – an integer value that indicates whether an I/O request is pending
(returns 1) or has been completed by Windows but not yet processed by SQL Server,
since it has not yet performed the context switch (returns 0); we know that the item is
pending for the SQL Server storage engine, as this is the purpose of the DMV
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