.\" ----------------------------------------------------------------------------
.\" "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: head/share/man/man4/md.4 287396 2015-09-02 14:08:43Z trasz $
.\"
.Dd October 30, 2007
.Dt MD 4
.Os
.Sh NAME
.Nm md
.Nd memory disk
.Sh SYNOPSIS
.Cd device md
.Sh DESCRIPTION
The
.Nm
driver provides support for four kinds of memory backed virtual disks:
.Bl -tag -width preload
.It Cm malloc
Backing store is allocated using
.Xr malloc 9 .
Only one malloc-bucket is used, which means that all
.Nm
devices with
.Cm 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
.Xr vmstat 8 .
.It Cm preload
A file loaded by
.Xr loader 8
with type
.Sq md_image
is used for backing store.
For backwards compatibility the type
.Sq mfs_root
is also recognized.
If the kernel is created with option
.Dv MD_ROOT
the first preloaded image found will become the root file system.
.It Cm vnode
A regular file is used as backing store.
This allows for mounting ISO images without the tedious
detour over actual physical media.
.It Cm 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
.Cm swap
backing is generally preferable over
.Cm malloc
backing.
.El
.Pp
For more information, please see
.Xr mdconfig 8 .
.Sh EXAMPLES
To create a kernel with a ramdisk or MD file system, your kernel config
needs the following options:
.Bd -literal -offset indent
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\\"
.Ed
.Pp
The image in
.Pa /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
.Xr mdconfig 8
man page.
Other tools will also create these images, such as NanoBSD.
.Sh SEE ALSO
.Xr gpart 8 ,
.Xr loader 8 ,
.Xr mdconfig 8 ,
.Xr mdmfs 8 ,
.Xr newfs 8 ,
.Xr vmstat 8
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 4.0
as a cleaner replacement
for the MFS functionality previously used in
.Tn PicoBSD
and in the
.Fx
installation process.
.Pp
The
.Nm
driver did a hostile takeover of the
.Xr vn 4
driver in
.Fx 5.0 .
.Sh AUTHORS
The
.Nm
driver was written by
.An Poul-Henning Kamp Aq Mt phk@FreeBSD.org .