an apparatus for controlling a configuration change in a storage system having a plurality of storage appliances with an i / o handler local to each storage appliance for transmitting and receiving i / o requests . a local volume mapper local to each storage appliance maps storage local to the storage appliance ; a remote volume mapper local to each storage appliance maps storage remote from the or each storage appliance ; and a spanning host interface layer responsive to detection of the configuration change receives and redirects i / o requests among the plurality of storage appliances .