The following objects are always reserved in the z15 HSA during POR, whether they are defined in the IOCDS for use: 

## 5.1.1 Multiple logical channel subsystems 

GLYPH<SM590000> Six CSSs 

GLYPH<SM590000> A total of 15 LPARs in each CSS0 to CSS4 

GLYPH<SM590000> A total of 10 LPARs in CSS5 

GLYPH<SM590000> Subchannel set 0 with 63.75 K devices in each CSS 

GLYPH<SM590000> Subchannel set 1 with 64 K minus one device in each CSS 

GLYPH<SM590000> Subchannel set 2 with 64 K minus one device in each CSS 

GLYPH<SM590000> Subchannel set 3 with 64 K minus one device in each CSS 

In the z/Architecture, a single channel subsystem  can have up to 256 channel paths that are defined, which limited the total numbers of I/O connectivity on older Z servers to 256. 

The introduction of multiple LCSSs  enabled an IBM Z server to have more than one channel subsystems logically, while each logical channel subsystem maintains the same manner of I/O processing. Also, a logical partition (LPAR) is now attached to a specific logical channel subsystem, which makes the extension of multiple logical channel subsystems not apparent to the operating systems and applications. The multiple image facility (MIF) in the structure enables resource sharing across LPARs within a single LCSS or across the LCSSs. 

The multiple LCSS structure extended the Z servers’ total number of I/O connectivity to support a balanced configuration for the growth of processor and I/O capabilities. 

A one-digit number ID starting from 0 (CSSID) is assigned to an LCSS, and a one-digit hexadecimal ID (MIF ID) starting from 0 is assigned to an LPAR within the LCSS. 

Note:  The phrase channel subsystem  has same meaning as logical channel subsystem  in this section, unless otherwise stated. 

## Subchannels 

A subchannel  provides the logical appearance of a device to the program and contains the information that is required for sustaining a single I/O operation. Each device is accessible by using one subchannel in a channel subsystem to which it is assigned according to the active IOCDS of the Z server. 

A subchannel set (SS) is a collection of subchannels within a channel subsystem. The maximum number of subchannels of a subchannel set determines how many devices are accessible to a channel subsystem. 

In z/Architecture, the first subchannel set of an LCSS can have 63.75 K subchannels (with 0.25 K reserved), with a subchannel set ID (SSID) of 0. By enabling the multiple subchannel sets, extra subchannel sets are available to increase the device addressability of a channel subsystem. For more information about multiple subchannel sets, see 5.1.2, “Multiple subchannel sets” on page 208. 

