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md(4) [freebsd man page]

MD(4)							   BSD Kernel Interfaces Manual 						     MD(4)

md -- memory disk SYNOPSIS
The md driver provides support for four kinds of memory backed virtual disks: malloc Backing store is allocated using malloc(9). Only one malloc-bucket is used, which means that all md devices with malloc backing must share the malloc-per-bucket-quota. The exact size of this quota varies, in particular with the amount of RAM in the system. The exact value can be determined with vmstat(8). preload A file loaded by loader(8) with type 'md_image' is used for backing store. For backwards compatibility the type 'mfs_root' is also recognized. If the kernel is created with option MD_ROOT the first preloaded image found will become the root file system. vnode A regular file is used as backing store. This allows for mounting ISO images without the tedious detour over actual physical media. swap Backing store 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 generally preferable over malloc backing. For more information, please see mdconfig(8). EXAMPLES
To create a kernel with a ramdisk or MD file system, your kernel config needs the following options: options MD_ROOT # MD is a potential root device options MD_ROOT_SIZE=8192 # 8MB ram disk makeoptions MFS_IMAGE=/h/foo/ARM-MD options ROOTDEVNAME="ufs:md0" The image in /h/foo/ARM-MD will be loaded as the initial image each boot. To create the image to use, please follow the steps to create a file-backed disk found in the mdconfig(8) man page. Other tools will also create these images, such as NanoBSD. SEE ALSO
disklabel(8), fdisk(8), loader(8), mdconfig(8), mdmfs(8), newfs(8), vmstat(8) HISTORY
The md driver first appeared in FreeBSD 4.0 as a cleaner replacement for the MFS functionality previously used in PicoBSD and in the FreeBSD installation process. The md driver did a hostile takeover of the vn(4) driver in FreeBSD 5.0. AUTHORS
The md driver was written by Poul-Henning Kamp <>. BSD
October 30, 2007 BSD

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

mdconfig -- configure and enable memory disks SYNOPSIS
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 DESCRIPTION
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. 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. -t type 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 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. -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) 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, kilo- byte, 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 construct- ing 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 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. EXAMPLES
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 /tmp: 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 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 md1 gnop create -o 512K md1 mount /dev/md1.nop /mnt SEE ALSO
md(4), ffs(7), bsdlabel(8), fdisk(8), mdmfs(8), malloc(9) HISTORY
The mdconfig utility first appeared in FreeBSD 5.0 as a cleaner replacement for the vn(4) and vnconfig(8) combo. AUTHORS
The mdconfig utility was written by Poul-Henning Kamp <>. BSD
November 30, 2013 BSD

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