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/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]
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 parame-
ters 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.
-d Detach a memory disk from the system and release all resources.
-r Resize a memory disk.
Select the type of the memory disk.
malloc Storage for this type of memory disk is allocated with malloc(9) . This lim-
its 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
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 pre-
ferred instead of using malloc backing.
null Bitsink; all writes do nothing, all reads return zeroes.
Filename to use for the vnode type memory disk. The -a and -t vnode options are
implied if not specified.
-l 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.
-n When printing md(4) device names, print only the unit number without the md(4) pre-
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 ter-
abyte respectively. The -a and -t swap options are implied if not specified.
Sectorsize to use for the memory disk, in bytes.
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 constructing bootable images for later down-
load 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 kernel.
Allocate and reserve all needed storage from the start, rather than as
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.
Request a specific unit number or device name for the md(4) device instead of auto-
matic allocation. If a device name is specified, it must be start with ``md'' fol-
lowed 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 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 md4
Detach and free all resources used by /dev/md4:
mdconfig -d -u md4
Create a 128MByte swap backed disk, initialize an ffs(7) file system on it, and mount it on
mdconfig -a -t swap -s 128M -u md10
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 md0
bsdlabel -w md0 auto
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 md1
gnop create -o 512K md1
mount /dev/md1.nop /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 replacement for the vn(4)
and vnconfig(8) combo.
The mdconfig utility was written by Poul-Henning Kamp <phk@FreeBSD.org>.
BSD November 30, 2013 BSD