"THE BEER-WARE LICENSE" (Revision 42):
<phk@FreeBSD.ORG> wrote this file. As long as you retain this notice you
can do whatever you want with this stuff. If we meet some day, and you think
this stuff is worth it, you can buy me a beer in return. Poul-Henning Kamp
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$FreeBSD: head/share/man/man4/md.4 63309 2000-07-17 13:13:04Z sheldonh $
.Dd July 4, 2000 .Dt MD 4 .Os .Sh NAME .Nm md .Nd memory disk .Sh SYNOPSIS .Cd device md
p To specify the default maximum size of .Xr malloc 9 backed disks: .Cd options MD_NSECT=20000 .Sh DESCRIPTION The .Nm driver provides support for two kinds of memory backed virtual disks: .Xr malloc 9 backed and pre-loaded module backed.
p The .Xr malloc 9 backed disks are severely limited in size by constraints imposed by .Xr malloc 9 . The limit imposed depends on the amount of physical RAM in the machine, and covers all .Xr malloc 9 backed instances of .Nm devices.
p A sector filled with identical bytes does not consume storage, and therefore the storage allocated by a .Nm disk can be freed with a command such as this: d -literal -offset indent dd if=/dev/zero of=/dev/md567 .Ed
p The default maximum size of a .Nm disk backed by .Xr malloc 9 is 20,000 sectors of 512 bytes each. This can be changed with the kernel option .Dv MD_NSECT .
p At boot time the .Nm driver will search for pre-loaded modules of type .Sq md_image and instantiate a .Nm device for each of these modules. The type .Sq mfs_root is also allowed for backward compatibility. These devices are backed by the RAM reserved by the .Xr loader 8 , and as such not limited by the .Xr malloc 9 size constraints. However, this also means that the storage cannot be released. For the same reason, the .Dq identical byte detection is not applicable.
p The .Nm driver uses the .Dq almost-clone convention, whereby opening device number N creates device instance number N+1.
p The .Xr vmstat 8 utility can be used to monitor memory usage by .Xr malloc 9 backed .Nm disks, but not those backed by pre-loaded modules. .Sh SEE ALSO .Xr disklabel 5 , .Xr disklabel 8 , .Xr fdisk 8 , .Xr loader 8 , .Xr newfs 8 , .Xr vmstat 8 .Sh HISTORY The .Nm driver first appeared in .Fx 4.0 as a cleaner replacement for the MFS functionality previously used in PicoBSD and in the FreeBSD installation process. .Sh AUTHORS The .Nm driver was written by .An Poul-Henning Kamp .Aq phk@FreeBSD.org .