IBM TS2340 User Guide - Page 33

Path Failover Single HBA and Multi-Port Devices Yet for another example, consider a single path from the HBA through the fabric to a multi-port device. This also provides a failover path unless the failure is with the HBA or the HBA's cable, at which point the connection is severed and a permanent path error occurs. Whereas, if the failure should occur on the device side there is still an alternative path available for the information to go through. Single HBA Mulit-Port Drive (DPF) Fabric X Library (CPF) Drive X Drive Library Controller Drive X Virtual Tape Library (DPF and CPF) a2500257 Figure 5. Single HBA and multi-port devices At startup or configuration, the system detects multiple logical devices of the tape drive. Each logical device is a physical path to the same tape drive. A backup and restore application can open and use only one logical device at a time because they represent the same physical device. Without path failover support, if a permanent path error occurs (because of an HBA or cable failure, for example), the application fails. It is possible to initiate manual failover by restarting the application on the alternate logical device, but the application has to be restarted from the beginning. A long back up or restore operation may have been in progress when the path error occurred. Sometimes manual failover may require operator intervention to reset the drive because a SCSI Reservation could still exist on the failing HBA path. When the path failover support is enabled on all logical devices, the device driver configures them internally as a single device with multiple paths. The application can still open and use only one logical device at a time. If an application opens the primary device and a permanent path error occurs, the device driver initiates failover error recovery automatically on an alternate path. If successful, the current operation continues on an alternate path without interrupting the application. The data path failover error recovery first restores the previous device state, SCSI Reservation, and tape position, and then retries the failing operation. Chapter 2. Common Extended Features 15