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correct action (including calling for service) when it detects that the POWER Hypervisor firmware lost control. The POWER Hypervisor can also request a service processor repair action, if necessary. 

## 3.1.3 Cross-cluster communication 

In the DS8900F, the I/O enclosures are connected point-to-point and each CPC uses a PCIe architecture. DS8900F uses the PCIe paths between the I/O enclosures to provide the cross-cluster (XC) communication between CPCs. This configuration means that no separate path is between the XC communications and I/O traffic, which simplifies the topology. During normal operations, the XC communication traffic uses a small portion of the overall available PCIe bandwidth (less than 1.7%), so XC communication traffic has a negligible effect on I/O performance. 

Figure 3-1 shows the redundant PCIe fabric design for XC communication in the DS8900F and depicts the Single Chip Modules (SCM #0 and SCM#1) in each CPC. If the I/O enclosure that is used as the XC communication path fails, the system automatically uses an available alternative I/O enclosure for XC communication. 

<img src='content_image/991361.jpg'>

## 3.1.4 Environmental monitoring 

The environment (power, fans, and temperature) is monitored by the FSP. Environmental critical and non-critical conditions generate emergency power-off warning (EPOW) events. 

Critical events (for example, a complete input power loss) trigger the correct signals from the hardware to start an emergency shutdown to prevent data loss without operating system or firmware involvement. Non-critical environmental events are logged and reported by using Event Scan. 

The temperature is also monitored. If the ambient temperature rises above a preset operating range, the rotational speed of the cooling fans increases. Temperature monitoring also warns the Licensed Internal Code (LIC) of potential environmental problems. An orderly system shutdown, including a service call to IBM, occurs when the operating temperature exceeds a critical level. 

