malloc  Storage for this type of memory disk is allocated with
                     malloc(9).  This limits the size to the malloc bucket
                     limit in the kernel.  If the -o reserve option is not
                     set, creating and filling a large malloc-backed memory
                     disk is a very easy way to panic the system.

             swap    Storage for this type of memory disk is allocated from
                     buffer memory.  Pages get pushed out to swap when the
                     system is under memory pressure, otherwise they stay in
                     the operating memory.  Using swap backing is generally
                     preferred instead of using malloc backing.