Below is a script to see the number of pending disk IO. The ideal value is 0. This value indicates the number of pending I/Os that are waiting to be completed. Each scheduler has a list of pending I/Os that are checked to determine whether they have been completed every time there is a context switch. The count is incremented when the request is inserted. This count is decremented when the request is completed.
SELECT SUM(pending_disk_io_count) AS [Number of pending I/Os]
FROM sys.dm_os_schedulers
