How are new LUNs discovered by an ESXi Host?
A manual rescan operation is required every time a new LUN is added
ESXi hosts automatically detects new LUNs as they are presented
A rescan operation is periodically performed by ESXi hosts to detect any new LUNs
A reboot operation is required to detect newly presented LUNs
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