md man page on FreeBSD

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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=\"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.

     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
     installation 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			       October 30, 2007				   BSD

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