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MDCONFIG(8)		FreeBSD	System Manager's Manual		   MDCONFIG(8)

     mdconfig -- configure and enable memory disks

     mdconfig -a -t type [-n] [-o [no]option] ... [-f file] [-s	size]
	      [-S sectorsize] [-u unit]	[-x sectors/track] [-y heads/cyl]
     mdconfig -d -u unit
     mdconfig -l [-n] [-v] [-u unit]

     The mdconfig utility configures and enables md(4) devices.

     Options indicate an action	to be performed:

     -a	     Attach a memory disk.  This will configure	and attach a memory
	     disk with the parameters specified	and attach it to the system.

     -d	     Detach a memory disk from the system and release all resources.

     -t	type
	     Select the	type of	the memory disk.

	     malloc   Storage for this type of memory disk is allocated	with
		      malloc(9).  This limits the size to the malloc bucket
		      limit in the kernel.  If the -o reserve option is	not
		      set, creating and	filling	a large	malloc-backed memory
		      disk is a	very easy way to panic a system.

	     vnode    A	file specified with -f file becomes the	backing	store
		      for this memory disk.

	     swap     Storage for this type of memory disk is allocated	from
		      buffer memory.  Pages get	pushed out to the swap when
		      the system is under memory pressure, otherwise they stay
		      in the operating memory.	Using swap backing is gener-
		      ally preferable over malloc backing.

     -f	file
	     Filename to use for the vnode type	memory disk. Options -a	and -t
	     vnode are implied if not specified.

     -l	     List configured devices.  If given	with -u, display details about
	     that particular device.  If -v option specified, show all

     -n	     When printing md device names, print only the unit	number without
	     the md prefix.

     -s	size
	     Size of the memory	disk.  Size is the number of 512 byte sectors
	     unless suffixed with a b, k, m, g,	or t which denotes byte, kilo-
	     byte, megabyte, gigabyte and terabyte respectively. Options -a
	     and -t swap are implied if	not specified.

     -S	sectorsize
	     Sectorsize	to use for malloc backed device.

     -x	sectors/track
	     See the description of the	-y option below.

     -y	heads/cylinder
	     For malloc	or vnode backed	devices, the -x	and -y options can be
	     used to specify a synthetic geometry.  This is useful for con-
	     structing bootable	images for later download to other devices.

     -o	[no]option
	     Set or reset options.

		     For vnode backed devices: avoid IO_SYNC for increased
		     performance but at	the risk of deadlocking	the entire

		     Allocate and reserve all needed storage from the start,
		     rather than as needed.

		     Enable clustering on this disk.

		     Enable/Disable compression	features to reduce memory

		     Disable/Enable extra sanity checks	to prevent the user
		     from doing	something that might adversely affect the sys-

		     Enable/Disable readonly mode.

     -u	unit
	     Request a specific	unit number for	the md(4) device instead of
	     automatic allocation.

     To	create a 4 megabyte malloc(9) backed memory disk.  The name of the
     allocated unit will be output on stdout like ``md3'':

	   mdconfig -a -t malloc -s 4m

     To	create a disk named /dev/md4 with /tmp/boot.flp	as backing storage:

	   mdconfig -a -t vnode	-f /tmp/boot.flp -u 4

     To	detach and free	all resources used by /dev/md4:

	   mdconfig -d -u 4

     To	create a 128MByte swap backed disk, initialize an ffs(7) file system
     on	it, and	mount it on /tmp:

	   mdconfig -a -t swap -s 128M -u 10
	   newfs -U /dev/md10
	   mount /dev/md10 /tmp
	   chmod 1777 /tmp

     To	create a 5MB file-backed disk (-a and -t vnode are implied):

	   dd if=/dev/zero of=somebackingfile bs=1k count=5k
	   mdconfig -f somebackingfile -u 0
	   bsdlabel -w md0 auto
	   newfs md0c
	   mount /dev/md0c /mnt

     To	create an md(4)	device out of an ISO 9660 CD image file	(-a and	-t
     vnode are implied), using the first available md(4) device, and then
     mount the new memory disk:

	   mount -t cd9660 /dev/`mdconfig -f cdimage.iso` /mnt

     md(4), ffs(7), bsdlabel(8), fdisk(8), mdmfs(8), malloc(9)

     The mdconfig utility first	appeared in FreeBSD 5.0	as a cleaner replace-
     ment for the vn(4)	and vnconfig(8)	combo.

     The mdconfig utility was written by Poul-Henning Kamp <>.

FreeBSD	10.2		       November	11, 2007		  FreeBSD 10.2


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