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MD(4)			     Kernel Interfaces Manual			   MD(4)

NAME
     md -- memory disk

SYNOPSIS
     To compile this driver into the kernel, place the following lines in your
     kernel configuration file:

	   device md

     Alternatively, to load the driver as a module at boot time, place the
     following line in loader.conf(5):

	   geom_md_load="YES"

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 module loaded by loader(8) with  type  `md_image'  is  used  for
	      backing store.  For backwards compatibility the type `mfs_root' is
	      also recognized.	See the description of module loading directives
	      in  loader.conf(5) and note that the module name will either be an
	      absolute path to the image file or the  name  of	a  file  in  the
	      module_path.

	      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_READONLY	   # disallow mounting root writeable
	   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.

ARM KERNEL OPTIONS
     On  the  armv7  architecture, an MD_ROOT image larger than approximately 55
     MiB may require building a custom kernel using several tuning  options  re-
     lated to kernel memory usage.

     options LOCORE_MAP_MB=<num>
	     This configures how much memory is mapped for the kernel during the
	     early  initialization  stages.  The value must be at least as large
	     as the kernel plus all preloaded modules, including the root image.
	     There is no downside to setting this value too large, as long as it
	     does not exceed the amount of physical memory.  The default  is  64
	     MiB.

     options NKPT2PG=<num>
	     This  configures the number of kernel L2 page table pages to preal-
	     locate during kernel initialization.  Each L2 page can map 4 MiB of
	     kernel space.  The value must be large enough  to	map  the  kernel
	     plus  all preloaded modules, including the root image.  The default
	     value is 32, which is sufficient to map 128 MiB.

     options VM_KMEM_SIZE_SCALE=<num>
	     This configures the amount of kernel virtual address (KVA) space to
	     dedicate to the kmem_arena map.  The scale value is  the  ratio  of
	     physical to virtual pages.  The default value of 3 allocates a page
	     of  KVA for each 3 pages of physical ram in the system.  The kernel
	     and modules, including the root image, also consume KVA.  The  com-
	     bination of a large root image and the default scaling may preallo-
	     cate  so  much KVA that there is not enough remaining address space
	     to allocate kernel stacks, IO buffers, and other resources that are
	     not part of kmem_arena.  Overallocating kmem_arena space is  likely
	     to  manifest  as  failure to launch userland processes with "cannot
	     allocate kernel stack" messages.  Setting the scale value too  high
	     may  result in kernel failure to allocate memory because kmem_arena
	     is too small, and the failure may require	significant  runtime  to
	     manifest.	 Empirically, a value of 5 works well for a 200 MiB root
	     image on a system with 2 GiB of physical ram.

SEE ALSO
     gpart(8), loader(8), mdconfig(8), mdmfs(8), newfs(8), vmstat(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 driver in FreeBSD 5.0.

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

FreeBSD ports 15.1		  July 16, 2025 			   MD(4)

Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=md&sektion=4&manpath=FreeBSD+15.1-RELEASE+and+Ports>

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