8456ch_RAS.fm Draft Document for Review January 13, 2020 5:02 pm 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. 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. 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. 70  IBM DS8900F Architecture and Implementation 3.1.4 Environmental monitoring Figure 3-1 DS8900F XC communication through the PCIe fabric and I/O enclosures 3.1.3 Cross-cluster communication  bay07 bay06 bay01 bay02 bay03 bay04 bay05 bay00 P9 SCM #1 P9 SCM #0 C12-T1 C9-T1 C8-T1 C6-T1 C4-T1 C3-T2 C3-T1 C2-T1 C12-T1 C9-T1 C8-T1 C6-T1 C4-T1 C3-T2 C3-T1 C2-T1 P9 SCM #1 P9 SCM #0