MDCONFIG(8) BSD System Manager’s Manual MDCONFIG(8)


mdconfig — configure and enable memory disks


mdconfig −a −t type [−n] [

−o [
]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:


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


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

−t type

Select the type of the memory disk.


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.


A file specified with −f file becomes the backingstore for this memory disk.


swapspace is used to back this memory disk.

−f file

Filename to use for the vnode type memory disk.


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.


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 constructing bootable images for later download to other devices.

−o [

Set or reset options.


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


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 usage.


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


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 replacement for the vn(4) and vnconfig(8) combo.


The mdconfig utility was written by Poul-Henning Kamp ⟨ phk [AT]⟩ .

BSD March 10, 2004 BSD