IBM DS8700, Introduction And Planning Manual

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Logical volumes
A logical volume is the storage medium that is associated with a logical disk. It
typically resides on one or more hard disk drives.
For the storage unit, the logical volumes are defined at logical configuration time.
For count-key-data (CKD) servers, the logical volume size is defined by the device
emulation mode and model. For fixed block (FB) hosts, you can define each FB
volume (LUN) with a minimum size of a single block (512 bytes) to a maximum
size of 2
32
blocks or 16 TB.
A logical device that has nonremovable media has one and only one associated
logical volume. A logical volume is composed of one or more extents. Each extent
is associated with a contiguous range of addressable data units on the logical
volume.
Allocation, deletion, and modification of volumes
All extents of the ranks assigned to an extent pool are independently available for
allocation to logical volumes. The extents for a LUN or volume are logically
ordered, but they do not have to come from one rank and the extents do not have
to be contiguous on a rank. This construction method of using fixed extents to
form a logical volume in the DS8000 allows flexibility in the management of the
logical volumes. You can delete volumes, resize volumes, and reuse the extents of
those volumes to create other volumes, different sizes. One logical volume can be
deleted without affecting the other logical volumes defined on the same extent
pool.
Because the extents are cleaned after you delete a volume, it can take some time
until these extents are available for reallocation. The reformatting of the extents is a
background process.
There are two extent allocation methods used by the DS8000: rotate volumes and
storage pool striping (rotate extents).
Storage pool striping: extent rotation
The default storage allocation method is storage pool striping. The extents of a
volume can be striped across several ranks. The DS8000 keeps a sequence of ranks.
The first rank in the list is randomly picked at each power on of the storage
subsystem. The DS8000 tracks the rank in which the last allocation started. The
allocation of a first extent for the next volume starts from the next rank in that
sequence. The next extent for that volume is taken from the next rank in sequence,
and so on. The system rotates the extents across the ranks.
If you migrate an existing non-striped volume to the same extent pool with a
rotate extents allocation method, then the volume is "reorganized." If you add more
ranks to an existing extent pool, then the "reorganizing" existing striped volumes
spreads them across both existing and new ranks.
You can configure and manage storage pool striping using the DS Storage
Manager, DS CLI, and DS Open API. The default of the extent allocation method
(EAM) option that is allocated to a logical volume is now rotate extents. The rotate
extents option is designed to provide the best performance by striping volume
extents across ranks in extent pool.
Chapter 2. Hardware and features
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