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 .\" ----------------------------------------------------------------------------
8 .\" $FreeBSD: src/share/man/man4/md.4,v 1.3.2.4 2001/06/18 22:00:12 dd Exp $
19 To specify the default maximum size of
22 .Cd options MD_NSECT=20000
26 driver provides support for two kinds of memory backed virtual disks:
29 and pre-loaded module backed.
35 are severely limited in size by constraints imposed by
37 Specifically, only one malloc bucket is used, which means that all
41 backing must share the malloc per-bucket quota.
42 The exact size of this quota varies,
43 in particuar with the amount of RAM in the system.
44 The exact value can be determined with
47 A sector filled with identical bytes does not consume storage,
48 and therefore the storage allocated by a
50 disk can be freed with a command such as this:
51 .Bd -literal -offset indent
52 dd if=/dev/zero of=/dev/md567
55 The default maximum size of a
60 is 20,000 sectors of 512 bytes each.
62 with the kernel option
67 driver will search for pre-loaded modules of type
71 device for each of these modules.
74 is also allowed for backward compatibility.
75 These devices are backed by the RAM reserved by the
77 and as such not limited by the
80 However, this also means that the storage cannot be released.
84 detection is not applicable.
91 whereby opening device number N creates device instance number N+1.
95 utility can be used to monitor memory usage by
100 but not those backed by pre-loaded modules.
109 if [ -e /dev/md0 -a -e /dev/md0c ]; then
110 disklabel -r -w md0 auto && \\
111 newfs /dev/md0c && \\
112 mount /dev/md0c /tmp && \\
126 driver first appeared in
128 as a cleaner replacement
129 for the MFS functionality previously used in PicoBSD
132 installation process.
136 driver was written by
137 .An Poul-Henning Kamp
138 .Aq phk@FreeBSD.org .