SYNCHRONOUS TO SEMI-SYNCHRONOUS OUT-OF-BAND I/O

Handling I/O operations between a storage system and a host includes initiating a direct data transfer for each of the I/O operations that initially excludes other processes from using a CPU of the host, setting a first timer for each of the direct data transfers, converting at least some of the direct transfers to semi-synchronous I/O operations that release the CPU for use by other processes and transfer data directly between the storage system and the host in response to the first timer expiring prior to completion of a corresponding one of the direct data transfers, and setting a second timer that corresponds to an expected completion of the semi-synchronous I/O operation. The direct data transfers may exchange data between the host and cache memory of the storage system. The direct data transfers may be performed using a high speed connection between the storage system and the host.