Methods, systems, and apparatus, including computer programs encoded on computer storage media, for memory requests by a virtual machine. One of the methods includes initiating a migration process to move an application executing on a first device from the first device to a second device by copying pages of data, stored in a memory of the first device and used for the execution of the application, from the first device to the second device while continuing to execute the application on the first device, updating, by the first device, one or more bytes in at least one of the pages of data in response to executing the application on the first device during the migration process, stopping execution of the application on the first device, and copying the updated bytes from the first device to the second device to cause the second device to continue execution of the application.