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MD(4)			 BSD Kernel Interfaces Manual			 MD(4)

     md	-- memory disk

     device md

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

     malloc   Backing store is allocated using malloc(9).  Only	one malloc-
	      bucket is	used, which means that all md devices with malloc
	      backing 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
	      backing 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	mount-
	      ing ISO images without the tedious detour	over actual physical

     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).

     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=
     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.

     disklabel(5), disklabel(8), fdisk(8), loader(8), mdconfig(8), mdmfs(8),
     newfs(8), vmstat(8)

     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 in-
     stallation	process.

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

     The md driver was written by Poul-Henning Kamp <>.

BSD			       November	24, 2006			   BSD


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