MD(4) BSD Kernel Interfaces Manual MD(4)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>.
BSD October 30, 2007 BSD
Check Out this Related Man Page
MDMFS(8) BSD System Manager's Manual MDMFS(8)NAME
mdmfs, mount_mfs -- configure and mount an in-memory file system using the md(4) driver
SYNOPSIS
mdmfs [-DLlMNPStUX] [-a maxcontig] [-b block-size] [-c blocks-per-cylinder-group] [-d max-extent-size] [-E path-mdconfig] [-e maxbpg]
[-F file] [-f frag-size] [-i bytes] [-m percent-free] [-n rotational-positions] [-O optimization] [-o mount-options] [-p permissions]
[-s size] [-v version] [-w user:group] md-device mount-point
DESCRIPTION
The mdmfs utility is designed to be a work-alike and look-alike of the deprecated mount_mfs(8). The end result is essentially the same, but
is accomplished in a completely different way. The mdmfs utility configures an md(4) disk using mdconfig(8), puts a UFS file system on it
(unless -P was specified) using newfs(8), and mounts it using mount(8). It can handle geom_uzip(4) compressed disk images, as long as the
kernel supports this GEOM class. All the command line options are passed to the appropriate program at the appropriate stage in order to
achieve the desired effect.
By default, mdmfs creates a swap-based (MD_SWAP) disk with soft-updates enabled and mounts it on mount-point. It uses the md(4) device spec-
ified by md-device. If md-device is 'md' (no unit number), it will use md(4)'s auto-unit feature to automatically select an unused device.
Unless otherwise specified with one of the options below, it uses the default arguments to all the helper programs.
The following options are available. Where possible, the option letter matches the one used by mount_mfs(8) for the same thing.
-a maxcontig
Specify the maximum number of contiguous blocks that will be laid out before forcing a rotational delay (see the -d option).
-b block-size
The block size of the file system, in bytes.
-c blocks-per-cylinder-group
The number of blocks per cylinder group in the file system.
-D If not using auto-unit, do not run mdconfig(8) to try to detach the unit before attaching it.
-d max-extent-size
The file system may choose to store large files using extents. This parameter specifies the largest extent size that may be used.
It is presently limited to its default value which is 16 times the file system blocksize.
-E path-mdconfig
Use path-mdconfig as a location of the mdconfig(8) utility.
-e maxbpg
Indicate the maximum number of blocks any single file can allocate out of a cylinder group before it is forced to begin allocating
blocks from another cylinder group.
-F file
Create a vnode-backed (MD_VNODE) memory disk backed by file.
-f frag-size
The fragment size of the file system in bytes.
-i bytes
Number of bytes per inode.
-l Enable multilabel MAC on the new file system.
-L Show the output of the helper programs. By default, it is sent to /dev/null.
-M Create a malloc(9) backed disk (MD_MALLOC) instead of a swap-backed disk.
-m percent-free
The percentage of space reserved for the superuser.
-N Do not actually run the helper programs. This is most useful in conjunction with -X.
-n rotational-positions
The default number of rotational positions to distinguish.
-O optimization
Select the optimization preference; valid choices are space and time, which will optimize for minimum space fragmentation and minimum
time spent allocating blocks, respectively.
-o mount-options
Specify the mount options with which to mount the file system. See mount(8) for more information.
-P Preserve the existing file system; do not run newfs(8). This only makes sense if -F is specified to create a vnode-backed disk.
-p permissions
Set the file (directory) permissions of the mount point mount-point to permissions. The permissions argument can be in any of the
mode formats recognized by chmod(1). If symbolic permissions are specified, the operation characters ``+'' and ``-'' are interpreted
relative to the initial permissions of ``a=rwx''.
-S Do not enable soft-updates on the file system.
-s size
Specify the size of the disk to create. This only makes sense if -F is not specified. That is, this will work for the default swap-
backed (MD_SWAP) disks, and the optional (-M) malloc(9) backed disks (MD_MALLOC).
-t Turn on the TRIM enable flag for newfs(8). The md(4) device supports the BIO_DELETE command, enabling the TRIM on created filesystem
allows return of freed memory to the system pool.
-U Enable soft-updates on the file system. This is the default, and is accepted only for compatibility. It is only really useful to
negate the -S flag, should such a need occur.
-v version
Specify the UFS version number for use on the file system; it may be either 1 or 2. The default is derived from the default of the
newfs(8) command.
-w user:group
Set the owner and group to user and group, respectively. The arguments have the same semantics as with chown(8), but specifying just
a user or just a group is not supported.
-X Print what command will be run before running it, and other assorted debugging information.
The -F and -s options are passed to mdconfig(8) as -f and -s, respectively. The -a, -b, -c, -d, -e, -f, -i, -m and -n options are passed to
newfs(8) with the same letter; the -O option is passed to newfs(8) as -o. The -o option is passed to mount(8) with the same letter. See the
programs that the options are passed to for more information on their semantics.
EXAMPLES
Create and mount a 32 megabyte swap-backed file system on /tmp:
mdmfs -s 32m md /tmp
The same file system created as an entry in /etc/fstab:
md /tmp mfs rw,-s32m 2 0
Create and mount a 16 megabyte malloc-backed file system on /tmp using the /dev/md1 device; furthermore, do not use soft-updates on it and
mount it async:
mdmfs -M -S -o async -s 16m md1 /tmp
Create and mount a geom_uzip(4) based compressed disk image:
mdmfs -P -F foo.uzip -oro md.uzip /tmp/
Mount the same image, specifying the /dev/md1 device:
mdmfs -P -F foo.uzip -oro md1.uzip /tmp/
Configure a vnode-backed file system and mount its first partition, using automatic device numbering:
mdmfs -P -F foo.img mds1a /tmp/
COMPATIBILITY
The mdmfs utility, while designed to be compatible with mount_mfs(8), can be useful by itself. Since mount_mfs(8) had some silly defaults, a
``compatibility'' mode is provided for the case where bug-to-bug compatibility is desired.
Compatibility is enabled by starting mdmfs with the name mount_mfs or mfs (as returned by getprogname(3)). In this mode, the following
behavior, as done by mount_mfs(8), is duplicated:
o The file mode of mount-point is set by default to 01777 as if -p 1777 was given on the command line.
SEE ALSO md(4), fstab(5), mdconfig(8), mount(8), newfs(8)AUTHORS
Dima Dorfman
BSD September 4, 2011 BSD