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 July 4, 2000 11.Dt MD 4 12.Os 13.Sh NAME 14.Nm md 15.Nd memory disk 16.Sh SYNOPSIS 17.Cd device md 18.Pp 19To specify the default maximum size of 20.Xr malloc 9 21backed disks: 22.Cd options MD_NSECT=20000 23.Sh DESCRIPTION 24The 25.Nm 26driver provides support for two kinds of memory backed virtual disks: 27.Xr malloc 9 28backed 29and pre-loaded module backed. 30.Pp 31The 32.Xr malloc 9 33backed 34disks 35are severely limited in size by constraints imposed by 36.Xr malloc 9 . 37The limit imposed depends on the amount of physical RAM in the machine, 38and covers all 39.Xr malloc 9 40backed instances of 41.Nm 42devices. 43.Pp 44A sector filled with identical bytes does not consume storage, 45and therefore the storage allocated by a 46.Nm 47disk can be freed with a command such as this: 48.Bd -literal -offset indent 49dd if=/dev/zero of=/dev/md567 50.Ed 51.Pp 52The default maximum size of a 53.Nm 54disk 55backed by 56.Xr malloc 9 57is 20,000 sectors of 512 bytes each. 58This can be changed 59with the kernel option 60.Dv MD_NSECT . 61.Pp 62At boot time the 63.Nm 64driver will search for pre-loaded modules of type 65.Sq md_image 66and instantiate a 67.Nm 68device for each of these modules. 69The type 70.Sq mfs_root 71is also allowed for backward compatibility. 72These devices are backed by the RAM reserved by the 73.Xr loader 8 , 74and as such not limited by the 75.Xr malloc 9 76size constraints. 77However, this also means that the storage cannot be released. 78For the same reason, 79the 80.Dq identical byte 81detection is not applicable. 82.Pp 83The 84.Nm 85driver uses the 86.Dq almost-clone 87convention, 88whereby opening device number N creates device instance number N+1. 89.Pp 90The 91.Xr vmstat 8 92utility can be used to monitor memory usage by 93.Xr malloc 9 94backed 95.Nm 96disks, 97but not those backed by pre-loaded modules. 98.Sh SEE ALSO 99.Xr disklabel 5 , 100.Xr disklabel 8 , 101.Xr fdisk 8 , 102.Xr loader 8 , 103.Xr newfs 8 , 104.Xr vmstat 8 105.Sh HISTORY 106The 107.Nm 108driver first appeared in 109.Fx 4.0 110as a cleaner replacement 111for the MFS functionality previously used in PicoBSD 112and in the FreeBSD installation process. 113.Sh AUTHORS 114The 115.Nm 116driver was written by 117.An Poul-Henning Kamp 118.Aq phk@FreeBSD.org . 119