Evaluating a potentially malicious sample using a copy-on-write overlay is disclosed. A first virtual machine instance is initialized as a copy-on-write overlay associated with an original virtual machine image. The first virtual machine image is started and a first sample is executed. A second virtual machine instance is initialized as a copy-on-write overlay associated with a second original virtual machine image. The second virtual machine image is started and a second sample is executed. The first and second samples are executed at an overlapping time.