| MD(4) | Device Drivers Manual | MD(4) | 
md —
pseudo-device md
md driver enables use of system or user memory as a
  disk. Memory for the disk must be allocated within the kernel or with
  mdconfig(8) before the
  md device may be used as a disk. Its behaviour is
  configured by the kernel options MEMORY_DISK_HOOKS,
  MEMORY_DISK_IS_ROOT,
  MEMORY_DISK_ROOT_SIZE,
  MEMORY_DISK_DYNAMIC, and
  MEMORY_DISK_SERVER.
| November 30, 2013 | NetBSD 10.1 |