This paper proposes simultaneous virtual-machine logging and replay. It performs logging and replay simultaneously on the same machine through the use of two virtual machines, one for the primary execution and the other for the backup execution. While the primary execution produces the execution history, the backup execution consumes the history by replaying it. The size of an execution log can be limited to a certain size; thus, huge storage devices becomes unnecessary. We developed such a logging and replaying feature in a VMM. It can log and replay the execution of the Linux operating system. Our experiment results show the overhead of the primary execution is only fractional.