Logical subsystems

To facilitate configuration, a storage unit partitions its possible logical volumes into groups of volumes. Each group is referred to as a logical subsystem (LSS).

Before you can allocate storage space as logical volumes for use by specific host systems, you must define disk groups. If you are allocating storage for zSeries host systems, the format for the volumes is count key data (CKD) and for open-system environments, the format is fixed block (FB).

As part of the storage configuration process, you can configure the maximum number of LSSs that you plan to use. The DS6000™ can contain up to 255 LSSs and each LSS can be connected to four other LSSs using a logical path. An LSS is a group of up to 256 logical volumes that have the same disk format, either CKD or FB. On zSeries hosts, an LSS is associated with a logical control unit image (LCU).

An LSS is uniquely identified within the storage unit by an identifier that consists two hex characters (0-9 or uppercase AF) for which the logical volumes are associated. A fully qualified LSS is designated using the storage unit identifier and the LSS identifier, such as IBM.1750-921-12FA123/1E. 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 unit.

The storage unit has a 64 KB 256 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 4 KB volume addresses.

Storage unit functions, including some that are associated with fixed-block logical volumes, might have dependencies on LSS partitions. For example:

The LSS partitions and their associated logical volume numbers must identify logical volumes that are specified for storage unit 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 unit.

If you increase storage unit 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 S/390 hosts or zSeries hosts, the storage unit does not process remote mirror and copy functions. The fixed-block LSS limit must be no lower then eight to support remote mirror and copy functions for open-systems hosts.