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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] [-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	backingstore
		      for this memory disk.

	     swap     swapspace	is used	to back	this memory disk.

     -f	file
	     Filename to use for the vnode type	memory disk.

     -l	     List information about configured md(4) devices.  If the -u
	     option is used in conjuction with this, the output	is limited to
	     information on the	specified device.

     -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 k, m, or g which denotes kilobyte,
	     megabyte and gigabyte respectively.

     -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:

	   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 and mount a 128MByte swap backed	file system 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:

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

     md(4), 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			March 10, 2004			  FreeBSD 10.2


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