 From MV Administrator guide.   "You cannot expand the capacity of a LUN or metaLUN that is participating in a MirrorView/S (MV/A) session until you destroy the mirror relationship. Neither the production nor secondary server can access this added capacity until the expansion is complete and you perform a full resynchronization. If possible, consider expanding the LUN before making it a mirror."    Note: No downtime on the production lun.     So you have to destroy the mirror relationship, expand both lun than create it again.     
New to the CX4 series, migrating from an AX4.  I am looking at expanding an existing LUN on a CX4-120 that is currently mirrored to another CX4-120, when clicking the expand option I get a message indicating that the LUN can not be expanded due to it being a being mirrored or in a storage group.   What are my options for expanding the LUN without downtime?
LUN Expansion - Clariion CX4-120 (flare 30)