IBM DS8000 User Manual page 61

After you create ranks, your physical storage can be considered virtualized.
Virtualization dissociates your physical storage configuration from your logical
configuration, so that volume sizes are no longer constrained by the physical size
of your arrays.
The available space on each rank is divided into extents. The extents are the
building blocks of the logical volumes. An extent is striped across all disks of an
array.
Extents of the same storage type are grouped together to form an extent pool.
Multiple extent pools can create storage classes that provide greater flexibility in
storage allocation through a combination of RAID types, DDM size, DDM speed,
and DDM technology. This allows a differentiation of logical volumes by assigning
them to the appropriate extent pool for the desired characteristics. Different extent
sizes for the same device type (for example, count-key-data or fixed block) can be
supported on the same storage unit, but these different extent types must be in
different extent pools.
A logical volume is composed of one or more extents. A volume group specifies a
set of logical volumes. By identifying different volume groups for different uses or
functions (for example, SCSI target, FICON/ESCON control unit, remote mirror
and copy secondary volumes, FlashCopy targets, and Copy Services), access to the
set of logical volumes that are identified by the volume group can be controlled.
Volume groups map hosts to volumes. Figure 7 on page 34 shows a graphic
representation of the logical configuration sequence.
When volumes are created, you must initialize logical tracks from the host before
the host is allowed read and write access to the logical tracks on the volumes. An
internal volume initialization process allows quicker access to logical volumes that
are used as host volumes and source volumes in Copy Services relationships, such
as FlashCopy or Remote Mirror and Copy relationships. Volumes are not available
as target volumes for Copy Services relationships until the volumes have been
fully initialized. After volume initialization is complete and successful, you can use
the target volumes to establish Copy Services relationships. Initialization requests
can fail if the initialization is performed on volumes that are participating in
existing Copy Services relationships.
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Chapter 1. Introduction to IBM System Storage DS8000 series