A computer system configured to provide fault tolerance includes a first host system and a second host system. The first host system is programmed to monitor a number of portions of memory of the first host system that have been modified by a guest running on the first host system and, upon determining that the number of portions exceeds a threshold level, determine that a checkpoint needs to be created. Upon determining that the checkpoint needs to be created, operation of the guest is paused and checkpoint data is generated. After generating the checkpoint data, operation of the guest is resumed while the checkpoint data is transmitted to the second host system.