You place a datastore in maintenance mode when you need to take it out of use to service it. A datastore enters or leaves maintenance mode only as the result of a user request.
Maintenance mode is available to datastores within a Storage DRS-enabled datastore cluster. Standalone datastores cannot be placed in maintenance mode.
Virtual disks that are located on a datastore that is entering maintenance mode must be migrated to another datastore, either manually or using Storage DRS. When you attempt to put a datastore in maintenance mode, the Placement Recommendations tab displays a list of migration recommendations, datastores within the same datastore cluster where virtual disks can be migrated. On the Faults tab, vCenter Server displays a list of the disks that cannot be migrated and the reasons why. If Storage DRS affinity or anti-affinity rules prevent disks from being migrated, you can choose to enable the Ignore Affinity Rules for Maintenance option.
The datastore is in a state of Entering Maintenance Mode until all virtual disks have been migrated.