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(5),
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⟩.