gtps4m18System Generation

Alternate Paths to Disk Storage

Sometimes when a program requests storage I/O, the pathway between the processor complex and disk storage is found to be busy because of an earlier I/O request on the same storage director (SD). When there is only 1 path defined through a storage director between the processor complex and disk storage, the program I/O request must wait for the earlier request to complete. When there is more than one path from a processor complex to disk storage, a program requesting storage I/O does not need to wait for an earlier request to complete. An alternate pathway can be selected automatically.

Pathways between the processor complex and disk storage are defined by the I/O configuration data set (IOCDS) for each installed processor. IOCDSs are created by an IBM-supplied I/O configuration program (IOCP) and are modified whenever there are any alterations required in the installed I/O configuration. For example, adding a new DASD controller would involve an addition to the IOCDS as part of the installation. The IOCP required for creating and maintaining IOCDSs is not a part of the TPF-supplied product. There is a separate IOCP associated with each IBM processor. For detailed IOCP operating information about a particular processor see ES/9000, ES/3090 Input/Output Configuration Program User's Guide and ESCON Channel-to-Channel Reference for the processor. For information about devices available on the System/370 processor, see IBM System/370 Input/Output Configurator.

The following are considerations that relate specifically to the use of alternate pathways to disk storage in TPF systems: