MDCONFIG(8) BSD 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 [-o [no]force]
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.
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.
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.
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.
Sectorsize to use for malloc backed device.
See the description of the -y option below.
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.
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.
Request a specific unit number for the md(4) device instead of
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
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 .
BSD June 21, 2008 BSD