NAME¶
md
—
memory disk
SYNOPSIS¶
device md
DESCRIPTION¶
The
md
driver provides support for four kinds
of memory backed virtual disks:
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 file system.
vnode
- A regular file is used as backing store. This allows for mounting ISO
images without the tedious detour over actual physical media.
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).
EXAMPLES¶
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
makeoptions MFS_IMAGE=/h/foo/ARM-MD
options ROOTDEVNAME=\"ufs:md0\"
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.
SEE ALSO¶
disklabel(8),
fdisk(8),
loader(8),
mdconfig(8),
mdmfs(8),
newfs(8),
vmstat(8)
HISTORY¶
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 installation process.
The
md
driver did a hostile takeover of the
vn(4) driver in
FreeBSD
5.0.
AUTHORS¶
The
md
driver was written by
Poul-Henning Kamp
⟨phk@FreeBSD.org⟩.