At present, two options exist for saving Julia data structures to disk: Julia's built-in serializer and the JLD (Julia data) package. The built-in serializer achieves reasonable performance, but uses a non-standardized format that differs by Julia version and processor architecture. JLD saves data structures in a standardized format (HDF5), but has substantial overhead when saving large numbers of mutable objects. In this talk, I describe the design of JLD2, a re-implementation of JLD. By replacing JLD's dependency on the HDF5 library with a pure Julia implementation of a subset of HDF5, JLD2 achieves performance comparable to Julia's built-in serializer, while writing files readable by standard HDF5 implementations. Additionally, JLD2 resolves numerous issues with the previous JLD format and implementation.