In the system storage hierarchy, if your goal is to create logical volumes, you begin with a disk drive module (DDM). Logical groupings of four DDMs form an array site. Logical groupings of one or two array sites form an array. When you define your array storage type as count key data, or CKD, or fixed block, you create a rank. A rank is divided into a number of fixed-size extents.
If you work with open-systems hosts, an extent is 1 GB. If you work with IBM System z™, zSeries or S/390® systems, then an extent is a CKD Mod 1 extent.
After you create extents, your physical storage is considered virtualized. Virtualization abstracts your physical storage configuration from your logical configuration, so that volume limits are no longer constrained by the physical size of your arrays.
Extents of the same storage type are grouped together to form an extent pool. Multiple extent pools can be used to create storage classes to provide greater flexibility in storage allocation through a combination of RAID types, disk size, disk speed and disk technology.
A volume is created from an extent pool. You can define your volumes using the storage available from the extents that are contained within the extent pool.