Sorry, I didn't quite answer your question! Yes, the expected behaviour would be for the disks to be initialized. Beyond that, it'd be a configuration setting

Sorry, I didn't quite answer your question! Yes, the expected behaviour would be for the disks to be initialized. Beyond that, it'd be a configuration setting for what to do with new disks. It could be used to extend the existing LUNs, or added as a new pair in a RAID 10 style setup, or added as new members of a different level RAID. Then it would be up to the sysadmin to extend the LUN, or divide the newly added space among multiple LUNs. Different arrays have different behaviour with new disk. Some only add to offline LUNs. Some do everything online with lower performance, as you said. Even some super expensive enterprise kit will be very easy to break by adding new disk.