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md - memory disk
The md driver provides support for four kinds of memory backed virtual
malloc Backing store is allocated using malloc(9). Only one malloc-
bucket is used, which means that all md devices with malloc
backing must share the malloc-per-bucket-quota. The exact size
of this quota varies, in particular with the amount of RAM in
the system. The exact value can be determined with vmstat(8).
preload A file loaded by loader(8) with type ‘md_image’ is used for
backing store. For backwards compatibility the type ‘mfs_root’
is also recognized. If the kernel is created with option
MD_ROOT the first preloaded image found will become the root
vnode A regular file is used as backing store. This allows for
mounting ISO images without the tedious detour over actual
swap Backing store is allocated from buffer memory. Pages get pushed
out to the swap when the system is under memory pressure,
otherwise they stay in the operating memory. Using swap backing
is generally preferable over malloc backing.
For more information, please see mdconfig(8).
To create a kernel with a ramdisk or MD file system, your kernel config
needs the following options:
options MD_ROOT #MD is a potential root device
options MD_ROOT_SIZE=8192 # 8MB ram disk
The image in /h/foo/ARM-MD will be loaded as the initial image each boot.
To create the image to use, please follow the steps to create a file-
backed disk found in the mdconfig(8) man page. Other tools will also
create these images, such as NanoBSD.
disklabel(5), disklabel(8), fdisk(8), loader(8), mdconfig(8), mdmfs(8),
The md driver first appeared in FreeBSD 4.0 as a cleaner replacement for
the MFS functionality previously used in PicoBSD and in the FreeBSD
The md driver did a hostile takeover of the vn(4) driver in FreeBSD 5.0.
The md driver was written by Poul-Henning Kamp 〈phk@FreeBSD.org〉.