Physically storage disks are divide in raid group. There is a virtualiation from raid group to ldev. Ldev is logical unit of storage. Only ldevs are refrenced by users. When an ldev is referenced it is first searched in cache if not located then actual raid group where data which is referenced is located. In xp-array there is an array of disk in a frame also called DKC.All DKC serial numbers are unique. There are multiple ports in each disk through which user access the disks. Two ports are maintained by one processor. Disk arrays are great power consumer.