.\" ----------------------------------------------------------------------------
.\" "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
.\" ----------------------------------------------------------------------------
.\"
.\" $FreeBSD$
.\"
.Dd March 9, 2001
.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 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 filesystem.
.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 swap space.
.El
.Pp
For more information, please see
.Xr mdconfig 8 .
.Sh SEE ALSO
.Xr disklabel 5 ,
.Xr disklabel 8 ,
.Xr fdisk 8 ,
.Xr loader 8 ,
.Xr mdconfig 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 phk@FreeBSD.org .