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 November 24, 2006 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 EXAMPLES 59To create a kernel with a ramdisk or MD file system, your kernel config 60needs the following options: 61.Bd -literal -ofset indent 62options MD_ROOT #MD is a potential root device 63options MD_ROOT_SIZE=8192 # 8MB ram disk 64makeoptions MFS_IMAGE=/h/foo/ARM-MD 65options ROOTDEVNAME=\"ufs:md0\" 66.Ed 67The image in 68.Pa /h/foo/ARM-MD 69will be loaded as the initial image each boot. 70To create the image to use, please follow the steps to create a file-backed 71disk found in the 72.Xr mdconfig 8 73man page. 74Other tools will also create these images, such as NanoBSD. 75.Sh SEE ALSO 76.Xr disklabel 5 , 77.Xr disklabel 8 , 78.Xr fdisk 8 , 79.Xr loader 8 , 80.Xr mdconfig 8 , 81.Xr mdmfs 8 , 82.Xr newfs 8 , 83.Xr vmstat 8 84.Sh HISTORY 85The 86.Nm 87driver first appeared in 88.Fx 4.0 89as a cleaner replacement 90for the MFS functionality previously used in 91.Tn PicoBSD 92and in the 93.Fx 94installation process. 95.Pp 96The 97.Nm 98driver did a hostile takeover of the 99.Xr vn 4 100driver in 101.Fx 5.0 . 102.Sh AUTHORS 103The 104.Nm 105driver was written by 106.An Poul-Henning Kamp 107.Aq phk@FreeBSD.org . 108