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Manual Pages  — MDCONFIG


mdconfig – create and control 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 creates and controls 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. If the -u unit option is not provided, the newly created device name will be printed on stdout.
  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 the 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 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.
null Bitsink; all writes do nothing, all reads return zeroes.
-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, t, or p which denotes byte, kilobyte, megabyte, gigabyte, terabyte and petabyte respectively. When used without the -r option, 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. This can be used with the -d flag to forcibly destroy an md(4) disk that is still in use.
[ no ] readonly
  Enable/disable readonly mode.
-u unit
  Request a specific unit number or device name for the md(4) device instead of automatic allocation. If a device name is specified, it must be start with "md" followed by the unit number.

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


Create a disk with /tmp/boot.flp as backing storage. The name of the allocated unit will be printed on stdout, such as "md0":
mdconfig /tmp/boot.flp

Create a 1 gigabyte swap backed memory disk named "md3":

mdconfig -s 1g -u md3

Detach and free all resources used by /dev/md3:

mdconfig -du md3

Show detailed information on current memory disks:

mdconfig -lv

Resize the "md3" memory disk to 2 gigabytes:

mdconfig -rs 2g -u md3

Create a 1 gigabyte swap backed disk, initialize an ffs(7) file system on it, and mount it on /tmp:

mdconfig -s 1g -u md10
newfs -U /dev/md10
mount /dev/md10 /tmp
chmod 1777 /tmp

Create a memory disk out of an ISO 9660 CD image file, using the first available md(4) device, and then mount it:

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


md(4), ffs(7), gpart(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 <Mt phk@FreeBSD.org>.

MDCONFIG (8) October 10, 2015

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