IBM DS8882F Introduction And Planning Manual page 24

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 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 configuration allows a differentiation of logical volumes by
assigning them to the appropriate extent pool for the needed 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. The 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. Identify different volume groups for different uses or
functions (for example, SCSI target, 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 4 on page 15 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. The
Quick Initialization feature for open system on FB ESE volumes allows quicker
access to logical volumes. The volumes include host volumes and source volumes
that can be used Copy Services relationships, such as FlashCopy or Remote Mirror
and Copy relationships. This process dynamically initializes logical volumes when
they are created or expanded, allowing them to be configured and placed online
more quickly.
You can specify LUN ID numbers through the graphical user interface (GUI) for
volumes in a map-type volume group. You can create a new volume group, add
volumes to an existing volume group, or add a volume group to a new or existing
host. Previously, gaps or holes in LUN ID numbers might result in a "map error"
status. The Status field is eliminated from the volume groups main page in the
GUI and the volume groups accessed table on the Manage Host Connections
page. You can also assign host connection nicknames and host port nicknames.
Host connection nicknames can be up to 28 characters, which is expanded from the
previous maximum of 12. Host port nicknames can be 32 characters, which are
expanded from the previous maximum of 16.
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DS8882F Introduction and Planning Guide