Multiple allegiance

With multiple allegiance, the storage unit can execute concurrent, multiple requests from multiple hosts.

Traditionally, IBM® storage subsystems allow only one channel program to be active to a disk volume at a time. This means that, once the subsystem accepts an I/O request for a particular unit address, this unit address appears "busy" to subsequent I/O requests. This single allegiance capability ensures that additional requesting channel programs cannot alter data that is already being accessed.

By contrast, the storage unit is capable of multiple allegiance (or the concurrent execution of multiple requests from multiple hosts). That is, the storage unit can queue and concurrently execute multiple requests for the same unit address, provided that no extent conflict occurs. A conflict refers to either the inclusion of a Reserve request by a channel program or a Write request to an extent that is in use.