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

create and control memory disks

mdconfig -a -t type [
] [
-o [
] ... [
-f file
] [
-s size
] [
-S sectorsize
] [
-u unit
] [
-x sectors/track
] [
-y heads/cylinder
] [
-L label

mdconfig -d -u unit [
-o [

mdconfig -r -u unit -s size [
-o [

mdconfig -l [
] [
] [
-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.
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.
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.
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.
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.
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.
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 download to other devices.
Associate a label (arbitrary string) with the new memory disk. The label can then be inspected with
mdconfig -l -v
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. This can be used with the -d flag to forcibly destroy an md(4) disk that is still in use.
Enable/disable readonly mode.
For vnode backed devices: enable/disable requesting verification of the file used for backing store. The type of verification depends on which security features are available. One example of verification is testing file integrity with checksums or cryptographic signatures.
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

open(2), 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 <>.
August 28, 2017 FreeBSD 12.0-RELEASE

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