How are new LUNs discovered by an ESXi host?
A rescan operation is periodically performed by ESXi hosts to detect new LUNs.
A reboot operation is required every time a LUN is added.
A manual rescan operation is required every time a LUN is added.
ESXi hosts automatically detect new LUNs as they are presented.
iSCSI SAN Configuration Guide, page 26.
The VMkernel discovers LUNs when it boots, and those LUNs are then visible in the vSphere Client. If changes are made to the LUNs, you must rescan to see those changes.
New LUNs created on the iSCSI storage
Changes to LUN access control
Changes in connectivity