1. A computer program product for executing an instruction to execute a Configure Storage Class Memory command in a system, said system having access to main storage and storage class memory, said computer program product comprising: a non-transitory computer readable storage medium readable by a processing circuit and storing instructions for execution by the processing circuit for performing a method comprising: obtaining by an input/output (I/O) subsystem a request block, the request block comprising a command code indicating the Configure Storage Class Memory command issued by a requesting configuration and a total size value to specify a requested number of increments of storage class memory to be configured for the requesting configuration; based on the command code, initiating a configuration process for configuring the storage class memory, the configuration process configured to allocate the requested number of increments of storage class memory specified in the total size value, wherein the initiating comprises performing one or more validity checks; continuing to perform the configuration process of the storage class memory based on the one or more validity checks being successful, wherein the one or more validity checks comprise determining that a total size requested does not exceed a number of storage class memory increments in an initialized state; commencing an asynchronous process to perform configuration of the storage class memory based on the one or more validity checks being successful; and providing a response code indicating whether the configuration process has been initiated.