Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for determining where to store a version of an object in an isolated environment. In one aspect, a method include actions of obtaining, from a process running in an isolated environment, a request to access an object and determining a frequency that changes to a version of the object stored in the isolated environment will be mapped back to a version of the object stored in the non-isolated environment. Additional actions include determining, based on the frequency that changes to a version of the object stored in the isolated environment are mapped back to a version of the object stored in the non-isolated environment, whether to store the version of the object in primary memory associated with the isolated environment or secondary memory associated with the isolated environment.