mdconfig —
configure MEMORY disks
  
    | mdconfig | special_file
      512-byte-blocks | 
The mdconfig command configures memory disk devices. It
  will associate the special file special_file with a
  range of user-virtual memory allocated by the mdconfig
  process itself. The mdconfig command should be run in
  the background. If successful, the command will not return. Otherwise, an
  error message will be printed.
To "unconfigure" the memory disk, just kill the
    background mdconfig process started earlier.
mdconfig /dev/md0c 2048
  &
Configures the memory disk md0c with one
    megabyte of user-space memory.
The special device will become inoperative if the
  mdconfig process is killed while the special device is
  open.