MDCONFIG(8) FreeBSD System Manager's Manual MDCONFIG(8)
NAME
mdconfig - create and control memory disks
SYNOPSIS
mdconfig -a -t type [-n] [-o [no]option] ... [-f file] [-s size]
[-S sectorsize] [-u unit] [-x sectors/track] [-y heads/cylinder]
[-L label]
mdconfig -d -u unit [-o [no]force]
mdconfig -r -u unit -s size [-o [no]force]
mdconfig -l [-n] [-v] [-f file] [-u unit]
mdconfig file
DESCRIPTION
The mdconfig utility creates and controls md(4) devices.
Options indicate an action to be performed:
-a Attach a memory disk. This will configure and attach a memory
disk with the parameters specified and attach it to the system.
If the -u unit option is not provided, the newly created device
name will be printed on stdout.
-d Detach a memory disk from the system and release all resources.
-r Resize a memory disk.
-t type
Select the type of the memory disk.
malloc Storage for this type of memory disk is allocated with
malloc(9). This limits the size to the malloc bucket
limit in the kernel. If the -o reserve option is not
set, creating and filling a large malloc-backed memory
disk is a very easy way to panic the system.
vnode A file specified with -f file becomes the backing store
for this memory disk.
swap Storage for this type of memory disk is allocated from
buffer memory. Pages get pushed out to swap when the
system is under memory pressure, otherwise they stay in
the operating memory. Using swap backing is generally
preferred instead of using malloc backing.
null Bitsink; all writes do nothing, all reads return zeroes.
-f file
Filename to use for the vnode type memory disk. The -a and -t
vnode options are implied if not specified.
-l List configured devices. If given with -u, display details about
that particular device. If given with -f file, display md(4)
device names of which file is used as the backing store. If both
of -u and -f options are specified, display devices which match
the two conditions. If the -v option is specified, show all
details.
-n When printing md(4) device names, print only the unit number
without the md(4) prefix.
-s size
Size of the memory disk. Size is the number of 512 byte sectors
unless suffixed with a b, k, m, g, t, or p which denotes byte,
kilobyte, megabyte, gigabyte, terabyte and petabyte respectively.
When used without the -r option, the -a and -t swap options are
implied if not specified.
-S sectorsize
Sectorsize to use for the memory disk, in bytes.
-x sectors/track
See the description of the -y option below.
-y heads/cylinder
For malloc or vnode backed devices, the -x and -y options can be
used to specify a synthetic geometry. This is useful for
constructing bootable images for later download to other devices.
-L label
Associate a label (arbitrary string) with the new memory disk.
The label can then be inspected with
mdconfig -l -v
-o [no]option
Set or reset options.
[no]async
For vnode backed devices: avoid IO_SYNC for increased
performance but at the risk of deadlocking the entire
kernel.
[no]cache
For vnode backed devices: enable/disable caching of data
in system caches. The default is to not cache.
Accesses via the device are converted to accesses via the
vnode. The caching policy for the vnode is used
initially. This is normally to cache. This caching
policy is retained if the cache option is used.
Otherwise, caching is limited by releasing data from
caches soon after each access. The release has the same
semantics as the POSIX_FADV_DONTNEED feature of
posix_fadvise(2). The result is that with normal (non-
zfs) caching, buffers are released from the buffer cache
soon after they are constructed, but their data is kept
in the page cache at lower priority.
The cache option tends to waste memory by giving unwanted
double caching, but it saves time if there is memory to
spare.
[no]reserve
Allocate and reserve all needed storage from the start,
rather than as needed.
[no]cluster
Enable clustering on this disk.
[no]compress
Enable/disable compression features to reduce memory
usage.
[no]force
Disable/enable extra sanity checks to prevent the user
from doing something that might adversely affect the
system. This can be used with the -d flag to forcibly
destroy an md(4) disk that is still in use.
[no]readonly
Enable/disable readonly mode.
[no]verify
For vnode backed devices: enable/disable requesting
verification of the file used for backing store. The
type of verification depends on which security features
are available. One example of verification is testing
file integrity with checksums or cryptographic
signatures.
-u unit
Request a specific unit number or device name for the md(4)
device instead of automatic allocation. If a device name is
specified, it must start with "md" followed by the unit number.
The last form, mdconfig file, is provided for convenience as an
abbreviation of mdconfig -a -t vnode -f file.
EXAMPLES
Create a disk with /tmp/boot.flp as backing storage. The name of the
allocated unit will be printed on stdout, such as "md0":
mdconfig /tmp/boot.flp
Create a 1 gigabyte swap backed memory disk named "md3":
mdconfig -s 1g -u md3
Detach and free all resources used by /dev/md3:
mdconfig -du md3
Show detailed information on current memory disks:
mdconfig -lv
Resize the "md3" memory disk to 2 gigabytes:
mdconfig -rs 2g -u md3
Create a 1 gigabyte swap backed disk, initialize an ffs(7) file system on
it, and mount it on /tmp:
mdconfig -s 1g -u md10
newfs -U /dev/md10
mount /dev/md10 /tmp
chmod 1777 /tmp
Create a memory disk out of an ISO 9660 CD image file, using the first
available md(4) device, and then mount it:
mount -t cd9660 /dev/`mdconfig -f cdimage.iso` /mnt
Create a file-backed device from a hard disk image that begins with 512K
of raw header information. gnop(8) is used to skip over the header
information, positioning md1.nop to the start of the filesystem in the
image.
mdconfig -u md1 -f diskimage.img
gnop create -o 512K md1
mount /dev/md1.nop /mnt
SEE ALSO
open(2), md(4), ffs(7), gpart(8), mdmfs(8), malloc(9)
HISTORY
The mdconfig utility first appeared in FreeBSD 5.0 as a cleaner
replacement for the vn kernel module and the vnconfig utility combo.
AUTHORS
The mdconfig utility was written by Poul-Henning Kamp <phk@FreeBSD.org>.
FreeBSD 13.1-RELEASE-p6 November 6, 2020 FreeBSD 13.1-RELEASE-p6
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