6947ch04.fm Draft Document for Review April 7, 2004 6:15 pm
110 IBM eServer zSeries 990 Technical Guide
4.1 Multiple Logical Channel Subsystem (LCSS)
The concept of Logical Channel Subsystem (LCSS) is new to the z990. The z990 supports up
to four Logical Channel Subsystems, hence the term multiple Logical Channel Subsystem.
The design of the z990 offers a considerable increase in processing power, memory sizes,
and I/O connectivity. In support of the larger I/O capability, the Channel Subsystem has been
scaled up correspondingly and the LCSS concept is designed to do just that. This concept is
introduced to facilitate the architectural change that provides more logical partitions and more
channels than before.
The structure provides up to four Logical Channel Subsystems. Each LCSS may have from 1
to 256 CHPIDs, and may in turn be configured with up to 15 logical partitions that relate to
that particular Logical Channel Subsystem. LCSSs are numbered from 0 to 3, and are
sometimes referred to as the CSS Image ID (CSSID 0, 1, 2, and 3).
Table4-1 shows the number of logical partitions and CHPIDs supported.
Table 4-1 Logical partitions and CHPID numbers support

4.1.1 Logical Channel Subsystem structure

The provision for multiple LCSSs is an extension to that provided on previous z/Architecture
servers. It provides channel connectivity to the defined logical partitions in a manner that is
transparent to subsystems and application programs.
The LCSS introduces new components and terminology that differs from previous server
generations. These components are explained in the following sections.

Logical Channel Subsystem (LCSS)

The z990 provides the ability to define more than 256 CHPIDs because of the introduction of
Logical Channel Subsystem concept. An LCSS is a logical replication of CSS facilities
(subchannels, CHPIDS, controls, and so on). This enables the definition of a balanced
configuration for the processor, and I/O capabilities. The LCSSs of the z990 introduce
significant changes to the I/O configuration.
For ease of management, it is strongly recommended that HCD be used to build and control
your z990 Input/Output Definition file (IODF). HCD support for z990 multiple logical channel
subsystems is available beginning with z/VM 4.4 and is available on all current OS/390 and
z/OS levels. HCD provides the capability to make both dynamic hardware and software I/O
configuration changes.
A z990 must have at least one LCSS defined. No logical partitions can exist without at least
one defined LCSS. Logical partitions are defined to an LCSS, not to a processor. Up to four
LCSSs are supported on the z990, and a logical partition is associated with one and only one
LCSS. CHPIDs are unique within an LCSS and range from 00 to FF. The same CHPID
number range is used again for the other LCSSs.
Note: The z990 provides for four Logical Channel Subsystems, 1024 CHPIDs, and up to
30 logical partitions for the total system.
LCSS
supported Number of
active LPARs Number of
defined LPARs Number of server
CHPIDs supported
LCSS 0, 1, 2, 3 30 30 1024