Aspects include synchronous input/output (I/O) replication of data. A synchronous I/O request is received from an operating system (OS) by firmware on a server. A plurality of synchronous I/O mailbox commands is transmitted by the firmware to a plurality of persistent storage control unit (SCU) nodes. The content of each of the plurality of synchronous I/O commands is based on the synchronous I/O request. A unit of work in the OS corresponding to the synchronous I/O request remains active at least until the synchronous I/O request is completed. It is determined, by the firmware, that determining that each of the plurality of persistent SCU nodes have completed their respective synchronous I/O commands. Based on the determining, the firmware indicates to the OS that the synchronous I/O request is completed.