IBM DS8900F, Introduction And Planning Manual

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LSSs using a logical path. An LSS is a group of up to 256 volumes that have the same storage type, either
count key data (CKD) for IBM Z hosts or fixed block (FB) for open systems hosts.
An LSS is uniquely identified within the storage system by an identifier that consists of two hex characters
(0-9 or uppercase A-F) for which the volumes are associated. A fully qualified LSS is designated using the
storage system identifier and the LSS identifier, such as IBM.2107-921-12FA123/1E. The LSS identifiers
are important for Copy Services operations. For example, for FlashCopy operations, you specify the LSS
identifier when choosing source and target volumes because the volumes can span LSSs in a storage
system.
The storage system has a 64K volume address space that is partitioned into 255 LSSs, where each LSS
contains 256 logical volume numbers. The 255 LSS units are assigned to one of 16 address groups, where
each address group contains 16 LSSs, or 4K volume addresses.
Storage system functions, including some that are associated with FB volumes, might have dependencies
on LSS partitions. For example:
• The LSS partitions and their associated volume numbers must identify volumes that are specified for
storage system Copy Services operations.
• To establish Remote Mirror and Copy pairs, a logical path must be established between the associated
LSS pair.
• FlashCopy pairs must reside within the same storage system.
If you increase storage system capacity, you can increase the number of LSSs that you have defined. This
modification to increase the maximum is a nonconcurrent action. If you might need capacity increases in
the future, leave the number of LSSs set to the maximum of 255.
Note: If you reduce the CKD LSS limit to zero for IBM Z hosts, the storage system does not process
Remote Mirror and Copy functions. The FB LSS limit must be no lower then eight to support Remote
Mirror and Copy functions for open-systems hosts.
Allocation methods
Allocation methods (also referred to as extent allocation methods) determine the means by which
provisioned capacity is allocated within a pool.
All extents of the ranks that are 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 storage system 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
that are 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 three allocation methods that are used by the storage system: rotate capacity (also referred to
as storage pool striping), rotate volumes, and managed.
Rotate capacity allocation method
The default allocation method is rotate capacity, which is also referred to as storage pool striping. The
rotate capacity allocation method is designed to provide the best performance by striping volume extents
across arrays in a pool. The storage system keeps a sequence of arrays. The first array in the list is
randomly picked at each power-on of the storage subsystem. The storage system tracks the array in
which the last allocation started. The allocation of a first extent for the next volume starts from the next
array 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 arrays.
If you migrate a volume with a different allocation method to a pool that has the rotate capacity allocation
method, then the volume is reallocated. If you add arrays to a pool, the rotate capacity allocation method
reallocates the volumes by spreading them across both existing and new arrays.
Chapter 1. Overview 17