Host transparent storage controller failover/failback of SCSI targets and associated units

A method of performing a failback process in a computer system having a pair of storage controllers in a coupled configuration, wherein one of the storage controllers has failed, each storage controller of the pair coupled to the other storage controller by a communication link and connected to a host CPU by a SCSI bus, the surviving storage controller having IDs and units associated therewith, comprising the steps of:

• monitoring by the surviving one of the controllers the status of the failed controller;
• detecting by the surviving controller when the failed controller has been replaced with a replacement controller, the replacement controller reestablishing communications with the surviving controller over the communication link;
• restarting the replacement controller;
• checking for a configuration information mismatch between the surviving controller and the replacement controller by comparing configuration information stored in the replacement controller to configuration information stored in the surviving controller;
• if a mismatch is detected, then copying the configuration information stored in the surviving controller to the replacement controller;
• determining whether or not the surviving controller has any write-back cache data for one or more of the associated units that were owned by the failed controller prior to failure;
• flushing the write-back cache data to the one or more of the associated units;
• and
• returning control of the one or more of the IDs and associated units belonging to the failed controller prior to failure to the replacement controller.

A method of performing a failover process in a computer system having a pair of storage controllers, each storage controller of the pair coupled to the other storage controller by a communication link and connected to a host CPU by a SCSI bus, each storage controller having units associated therewith, comprising the steps of:

• placing the storage controllers in a dual-active redundant configuration, wherein each controller serves one or more preferred SCSI IDs;
• sensing by one of the controllers in the dual-active redundant configuration that the other has failed;
• disabling the failed controller;
• and
• assuming control of the one or more preferred SCSI IDs of the failed controller by the surviving controller, whereby the surviving controller is capable of servicing the SCSI IDs of both of the controllers.