Variable partitioning in a hybrid memory subsystem

A memory module includes a volatile memory, a non-volatile memory, and a memory controller adapted to present to a host system external to the memory module an address space that includes an address space of the volatile memory and excludes all addresses of the non-volatile memory capacity. The module includes logic to copy the contents of the volatile memory to memory locations of the nonvolatile memory capacity reserved for backup of the volatile memory, using power from a backup power interface, when suitable power from the host system is unavailable, wherein the memory controller reserves for backup of the volatile memory an amount of nonvolatile memory storage capacity that is at least twice a memory storage capacity of the volatile memory.