Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for migrating data from a first storage system to a second storage system. That data is for a process (e.g., a virtual machine, application, or some other process) operating on a computer system that is separate from the first storage system and the second storage system. That data is stored according to data subsets that are each exclusive of each other. As each data set is migrated, access to the data set by the process is precluded, but access for the remaining data sets is not affected. Once the data migration is complete, access to the data set by the process is restored.