xref: /freebsd/share/man/man4/md.4 (revision c98323078dede7579020518ec84cdcb478e5c142)
1.\" ----------------------------------------------------------------------------
2.\" "THE BEER-WARE LICENSE" (Revision 42):
3.\" <phk@FreeBSD.ORG> wrote this file.  As long as you retain this notice you
4.\" can do whatever you want with this stuff. If we meet some day, and you think
5.\" this stuff is worth it, you can buy me a beer in return.   Poul-Henning Kamp
6.\" ----------------------------------------------------------------------------
7.\"
8.\" $FreeBSD$
9.\"
10.Dd March 9, 2001
11.Dt MD 4
12.Os
13.Sh NAME
14.Nm md
15.Nd memory disk
16.Sh SYNOPSIS
17.Cd device md
18.Sh DESCRIPTION
19The
20.Nm
21driver provides support for four kinds of memory backed virtual disks:
22.Bl -tag -width preload
23.It Cm malloc
24Backing store is allocated using
25.Xr malloc 9 .
26Only one malloc-bucket is used, which means that all
27.Nm
28devices with
29.Cm malloc
30backing must share the malloc-per-bucket-quota.
31The exact size of this quota varies, in particular with the amount
32of RAM in the
33system.
34The exact value can be determined with
35.Xr vmstat 8 .
36.It Cm preload
37A file loaded by
38.Xr loader 8
39with type
40.Sq md_image
41is used for backing store.
42For backwards compatibility the type
43.Sq mfs_root
44is also recognized.
45If the kernel is created with option
46.Dv MD_ROOT
47the first preloaded image found will become the root file system.
48.It Cm vnode
49A regular file is used as backing store.
50This allows for mounting ISO images without the tedious
51detour over actual physical media.
52.It Cm swap
53Backing store is allocated from swap space.
54.El
55.Pp
56For more information, please see
57.Xr mdconfig 8 .
58.Sh SEE ALSO
59.Xr disklabel 5 ,
60.Xr disklabel 8 ,
61.Xr fdisk 8 ,
62.Xr loader 8 ,
63.Xr mdconfig 8 ,
64.Xr mdmfs 8 ,
65.Xr newfs 8 ,
66.Xr vmstat 8
67.Sh HISTORY
68The
69.Nm
70driver first appeared in
71.Fx 4.0
72as a cleaner replacement
73for the MFS functionality previously used in
74.Tn PicoBSD
75and in the
76.Fx
77installation process.
78.Pp
79The
80.Nm
81driver did a hostile takeover of the
82.Xr vn 4
83driver in
84.Fx 5.0 .
85.Sh AUTHORS
86The
87.Nm
88driver was written by
89.An Poul-Henning Kamp
90.Aq phk@FreeBSD.org .
91