1. A memory module comprising: a volatile memory; a non-volatile memory; 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 a non-volatile memory capacity; a primary power supply input adapted to receive power from the host system; a backup power interface; and logic to copy contents of the volatile memory to memory locations of the non-volatile memory capacity reserved for backup of the volatile memory, using power from the 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 the non-volatile memory capacity that is at least twice a memory storage capacity of the volatile memory.