To support Redundant I/O Interconnect (RII) between domain pair 0 and 1, the two interconnects to each pair must be driven from two different PCIe fanouts. Normally, each PCIe interconnect in a pair supports the eight features in its domain. In backup operation mode, one PCIe interconnect supports all 16 features in the domain pair. 

Note:  The PCIe Interconnect (switch) adapter must  be installed in the PCIe+ I/O drawer to maintain the interconnect across I/O domains. If the adapter is removed (for a service operation), the I/O cards in that domain (up to eight) become unavailable. 

## 2.7.2 Enhanced drawer availability (EDA) 

With EDA, the effect of CPC drawer replacement is minimized. In a multiple CPC drawer system, a single CPC drawer can be concurrently removed and reinstalled for an upgrade or repair. Removing a CPC drawer without affecting the workload requires sufficient resources in the remaining CPC drawer. 

Before removing the CPC drawer, the contents of the PUs and memory of the drawer must be relocated. PUs must be available on the remaining CPC drawers to replace the deactivated drawer. Also, sufficient redundant memory must be available if no degradation of applications is allowed. To ensure that the CPC configuration supports removal of a CPC drawer with minimal effect on the workload, consider the flexible memory option. Any CPC drawer can be replaced, including the first CPC drawer that initially contains the HSA. 

Removal of a CPC drawer also removes the CPC drawer connectivity to the I/O drawers, PCIe I/O drawers, and coupling links. The effect of the removal of the CPC drawer on the system is limited by the use of redundant I/O interconnect. (For more information, see 2.7.1, “Redundant I/O interconnect” on page 70.) However, all ICA SR1.1 links that are installed in the removed CPC drawer must be configured offline. 

If the enhanced drawer availability and flexible memory options are not  used when a CPC drawer must be replaced, the memory in the failing drawer is also removed. This process might be necessary during an upgrade or a repair action. Until the removed CPC drawer is replaced, a power-on reset of the system with the remaining CPC drawers is supported. The CPC drawer can then be replaced and added back into the configuration concurrently. 

## 2.7.3 CPC drawer upgrade 

All fanouts that are used for I/O and coupling links are rebalanced concurrently as part of a CPC drawer addition to support better RAS characteristics. 

## 2.8 Model configurations 

When a z15 is ordered, the PUs are characterized according to their intended usage. The PUs can be ordered as any of the following items: 

CP The processor is purchased and activated. PU supports running the z/OS, z/VSE, z/VM, z/TPF, and Linux on Z$^{9}$ operating systems. It can also run Coupling Facility Control Code. 

IFL The Integrated Facility for Linux (IFL) is a processor that is purchased and activated for use by z/VM for Linux guests and Linux on Z$^{9}$ operating systems. 

