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MD(4)			     Kernel Interfaces Manual			   MD(4)

NAME
     md -- memory disk

SYNOPSIS
     device md

DESCRIPTION
     The  md  driver  provides	support  for four kinds of memory backed virtual
     disks:

     malloc   Backing store is allocated  using  malloc(9).   Only  one  malloc-
	      bucket  is used, which means that all md devices with malloc back-
	      ing must share the malloc-per-bucket-quota.   The  exact	size  of
	      this  quota  varies,  in	particular with the amount of RAM in the
	      system.  The exact value can be determined with vmstat(8).

     preload  A file loaded by loader(8) with type `md_image' is used for  back-
	      ing  store.   For  backwards  compatibility the type `mfs_root' is
	      also recognized.	If the kernel is created with option MD_ROOT the
	      first preloaded image found will become the root file system.

     vnode    A regular file is used as backing store.	This allows for mounting
	      ISO images without the tedious detour over actual physical media.

     swap     Backing store is allocated from buffer memory.  Pages  get  pushed
	      out  to  the swap when the system is under memory pressure, other-
	      wise they stay in the operating memory.	Using  swap  backing  is
	      generally preferable over malloc backing.

     For more information, please see mdconfig(8).

EXAMPLES
     To  create  a  kernel  with a ramdisk or MD file system, your kernel config
     needs the following options:

	   options	   MD_ROOT		   # MD is a potential root device
	   options	   MD_ROOT_SIZE=8192	   # 8MB ram disk
	   makeoptions	   MFS_IMAGE=/h/foo/ARM-MD
	   options	   ROOTDEVNAME=\"ufs:md0\"

     The image in /h/foo/ARM-MD will be loaded as the initial image  each  boot.
     To create the image to use, please follow the steps to create a file-backed
     disk found in the mdconfig(8) man page.  Other tools will also create these
     images, such as NanoBSD.

SEE ALSO
     disklabel(8),  fdisk(8),  loader(8),  mdconfig(8),  mdmfs(8), newfs(8), vm-
     stat(8)

HISTORY
     The md driver first appeared in FreeBSD 4.0 as a  cleaner	replacement  for
     the MFS functionality previously used in PicoBSD and in the FreeBSD instal-
     lation process.

     The md driver did a hostile takeover of the vn(4) driver in FreeBSD 5.0.

AUTHORS
     The md driver was written by Poul-Henning Kamp <phk@FreeBSD.org>.

FreeBSD 9.0			October 30, 2007			   MD(4)
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