A single copy of the data is maintained on switchable hardware; either an expansion unit (tower) or an IOP in a logical partition environment.
When an outage occurs on the primary node, access to the data on the switchable hardware switches to a designated backup node. Additionally, independent disk pools can be used in a cross-site mirroring (XSM) environment. This allows a mirror copy of the independent disk pool to be maintained on a system that is (optionally) geographically distant from the originating site for availability or protection purposes.
Careful planning is required if you plan to take advantage of switchable resources residing on switchable independent disk pools or cross-site mirroring (XSM).