10-06-2005
Virtual memory: The use of a disk partition or a file on disk to provide the same facilities usually provided by RAM. A way of using disk storage space to make the computer work as if it had more memory. When a file or program is too big for the computer to work with in its memory, part of the data is stored on disk. This virtual storage is divided into segments called pages; each page is correlated with a location in physical memory, or RAM. When an address is referenced, the page is swapped into memory; it is sent back to disk when other pages must be called. The program runs as if all the data is in memory.
RAM: Random Access Memory. The working memory of the computer. RAM is the memory used for storing data temporarily while working on it, running application programs, etc. "Random access" refers to the fact that any area of RAM can be accessed directly and immediately, in contrast to other media such as a magnetic tape where the tape must be wound to the point where the data is. RAM is called volatile memory; information in RAM will disappear if the power is switched off before it is saved to disk.
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MD(4) BSD Kernel Interfaces Manual MD(4)
NAME
md -- memory disk
SYNOPSIS
device md
DESCRIPTION
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(5), 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 <phk@FreeBSD.org>.
BSD
October 30, 2007 BSD