The Disk Writes Per Second alert provides the rate of write operations on the disk. Note that this alert is disk specific.
A high number of disk writes per second may indicate that the SQL Server is running low on allocated memory due to large amounts of data being written. It may also point to a situation of memory pressure that occurs when data is not able to remain in cache and has to be flushed to disk frequently.
This alert is raised based on the diagnostic data collection interval specified in Server Properties. Note that you need to enable this alert first in the Alert Configuration window to rise.
The Disk throughput chart provides a graphical analysis of the number of disk reads and writes over time.