A virtualized computer system provides fault tolerant operation of a primary virtual machine. In one embodiment, this system includes a backup computer system that stores a snapshot of the primary virtual machine and a log file containing non-deterministic events occurring in the instruction stream of the primary virtual machine. The primary virtual machine periodically updates the snapshot and the log file. Upon a failure of the primary virtual machine, the backup computer can instantiate a failover backup virtual machine by consuming the stored snapshot and log file.