The refresh operation for datastores, storage devices, and storage adapters updates the lists and storage information displayed in the vSphere Web Client. For example, it updates such information as the datastore capacity. When you perform storage management tasks or make changes in the SAN configuration, you might need to rescan your storage.
When you perform VMFS datastore management operations, such as creating a VMFS datastore or RDM, adding an extent, and increasing or deleting a VMFS datastore, your host or the vCenter Server automatically rescans and updates your storage. You can disable the automatic rescan feature by turning off the Host Rescan Filter. See Turn Off Storage Filters.
In certain cases, you need to perform a manual rescan. You can rescan all storage available to your host, or, if you are using the vCenter Server, to all hosts in a folder, cluster, and datacenter.
If the changes you make are isolated to storage connected through a specific adapter, perform a rescan for this adapter.
Perform the manual rescan each time you make one of the following changes.
Zone a new disk array on a SAN.
Create new LUNs on a SAN.
Change the path masking on a host.
Reconnect a cable.
Change CHAP settings (iSCSI only).
Add or remove discovery or static addresses (iSCSI only).
Add a single host to the vCenter Server after you have edited or removed from the vCenter Server a datastore shared by the vCenter Server hosts and the single host.
If you rescan when a path is unavailable, the host removes the path from the list of paths to the device. The path reappears on the list as soon as it becomes available and starts working again.