vn (4)

Leading comments

----------------------------------------------------------------------------
"THE BEER-WARE LICENSE" (Revision 42):
<phk@FreeBSD.org> wrote this file.  As long as you retain this notice you
can do whatever you want with this stuff. If we meet some day, and you think
this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
----------------------------------------------------------------------------

$FreeBSD: releng/11.0/share/man/man4/md.4 287396 2015-09-02 14:08:43Z trasz $

(The comments found at the beginning of the groff file "man4/vn.4freebsd".)

NAME

md - memory disk

SYNOPSIS

device md

DESCRIPTION

The 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 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 driver first appeared in Fx 4.0 as a cleaner replacement for the MFS functionality previously used in PicoBSD and in the Fx installation process.

The driver did a hostile takeover of the vn(4) driver in Fx 5.0 .

AUTHORS

The driver was written by An Poul-Henning Kamp Aq phk@FreeBSD.org .