You can automate the management of all of your datastore cluster resources using the Storage DRS. In a datastore cluster, a Storage DRS generates recommendations for migrating and placing the disks of your virtual machines to different datastores based on the availability of I/O resources and free space. DRS-enabled storage automates the load balancing of space across different datastores of a cluster. It also does the load balancing of I/O load of a virtual machine's disks and places their disks accordingly to satisfy their I/O needs.
Failed DRS recommendations
When automated, Storage DRS tries to implement the recommendations for low space or I/O load balancing; but sometimes it doesn't work and fails to apply recommendations. ...