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


mdconfigconfigure 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/cylinder]

mdconfig -d -u unit [ -o [ no] force]

mdconfig -r -u unit -s size [ -o [ no] force]

mdconfig -l [ -n][ -v][ -f file][ -u unit]

mdconfig file


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.
Resize a memory disk.
-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 backing store for this memory disk.
Storage for this type of memory disk is allocated from buffer memory. Pages get pushed out to swap when the system is under memory pressure, otherwise they stay in the operating memory. Using swap backing is generally preferred instead of using malloc backing.
-f file
Filename to use for the vnode type memory disk. The -a and -t vnode options are implied if not specified.
List configured devices. If given with -u, display details about that particular device. If given with -f file, display md(4) device names of which file is used as the backing store. If both of -u and -f options are specified, display devices which match the two conditions. If the -v option is specified, show all details.
When printing md(4) device names, print only the unit number without the md(4) 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, kilobyte, megabyte, gigabyte and terabyte respectively. The -a and -t swap options are implied if not specified.
-S sectorsize
Sectorsize to use for the memory disk, in bytes.
-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 [ no] option
Set or reset options.
[ no] async
For vnode backed devices: avoid IO_SYNC for increased performance but at the risk of deadlocking the entire kernel.
[ no] reserve
Allocate and reserve all needed storage from the start, rather than as needed.
[ no] cluster
Enable clustering on this disk.
[ no] compress
Enable/disable compression features to reduce memory usage.
[ no] force
Disable/enable extra sanity checks to prevent the user from doing something that might adversely affect the system.
[ no] readonly
Enable/disable readonly mode.
-u unit
Request a specific unit number for the md(4) device instead of automatic allocation.

The last form, mdconfig file, is provided for convenience as an abbreviation of mdconfig -a -t vnode -f file.


Create a 4 megabyte malloc(9) backed memory disk. The name of the allocated unit will be printed on stdout, such as “ md3”:

mdconfig -a -t malloc -s 4m

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

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

Detach and free all resources used by /dev/md4:

mdconfig -d -u 4

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

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

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 

Create a file-backed device from a hard disk image that begins with 512K of raw header information. gnop(8) is used to skip over the header information, positioning md1.nop to the start of the filesystem in the image.

mdconfig -f diskimage.img -u 1 
gnop create -o 512K md1 
mount /dev/md1.nop /mnt


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@FreeBSD.org>.
June 20, 2013 FreeBSD