xref: /dragonfly/share/man/man4/md.4 (revision cc93b0eb) 
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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
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7.\"
8.\" $FreeBSD: src/share/man/man4/md.4,v 1.3.2.4 2001/06/18 22:00:12 dd Exp $
9.\" $DragonFly: src/share/man/man4/md.4,v 1.5 2007/11/07 21:38:00 swildner Exp $
10.\"
11.Dd July 4, 2000
12.Dt MD 4
13.Os
14.Sh NAME
15.Nm md
16.Nd memory disk
17.Sh SYNOPSIS
18.Cd device md
19.Pp
20To specify the default maximum size of
21.Xr kmalloc 9
22backed disks:
23.Cd options MD_NSECT=20000
24.Sh DESCRIPTION
25The
26.Nm
27driver provides support for two kinds of memory backed virtual disks:
28.Xr kmalloc 9
29backed
30and pre-loaded module backed.
31.Pp
32The
33.Xr kmalloc 9
34backed
35disks
36are severely limited in size by constraints imposed by
37.Xr kmalloc 9 .
38Specifically, only one malloc bucket is used, which means that all
39.Nm
40devices with
41.Xr kmalloc 9
42backing must share the malloc per-bucket quota.
43The exact size of this quota varies,
44in particular with the amount of RAM in the system.
45The exact value can be determined with
46.Xr vmstat 8 .
47.Pp
48A sector filled with identical bytes does not consume storage,
49and therefore the storage allocated by a
50.Nm
51disk can be freed with a command such as this:
52.Bd -literal -offset indent
53dd if=/dev/zero of=/dev/md567
54.Ed
55.Pp
56The default maximum size of a
57.Nm
58disk
59backed by
60.Xr kmalloc 9
61is 20,000 sectors of 512 bytes each.
62This can be changed
63with the kernel option
64.Dv MD_NSECT .
65.Pp
66At boot time the
67.Nm
68driver will search for pre-loaded modules of type
69.Sq md_image
70and instantiate a
71.Nm
72device for each of these modules.
73The type
74.Sq mfs_root
75is also allowed for backward compatibility.
76These devices are backed by the RAM reserved by the
77.Xr loader 8 ,
78and as such not limited by the
79.Xr kmalloc 9
80size constraints.
81However, this also means that the storage cannot be released.
82For the same reason,
83the
84.Dq identical byte
85detection is not applicable.
86.Pp
87The
88.Nm
89driver uses the
90.Dq almost-clone
91convention,
92whereby opening device number N creates device instance number N+1.
93.Pp
94The
95.Xr vmstat 8
96utility can be used to monitor memory usage by
97.Xr kmalloc 9
98backed
99.Nm
100disks,
101but not those backed by pre-loaded modules.
102.Sh EXAMPLES
103To mount a
104.Xr kmalloc 9
105backed
106.Nm
107device on
108.Pa /tmp :
109.Bd -literal
110if [ -e /dev/md0s0 -a -e /dev/md0s0c ]; then
111	disklabel -r -w md0s0 auto && \\
112	newfs /dev/md0s0c && \\
113	mount /dev/md0s0c /tmp && \\
114	chmod 1777 /tmp
115fi
116.Ed
117.Sh SEE ALSO
118.Xr disklabel 5 ,
119.Xr disklabel 8 ,
120.Xr fdisk 8 ,
121.Xr loader 8 ,
122.Xr newfs 8 ,
123.Xr vmstat 8
124.Sh HISTORY
125The
126.Nm
127driver first appeared in
128.Fx 4.0
129as a cleaner replacement
130for the MFS functionality previously used in PicoBSD
131and in the
132.Fx
133installation process.
134.Sh AUTHORS
135The
136.Nm
137driver was written by
138.An Poul-Henning Kamp
139.Aq phk@FreeBSD.org .
140