create and control memory disks
utility creates and controls
Options indicate an action to be performed:
- 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
- Detach a memory disk from the system and release all
- Resize a memory disk.
- Select the type of the memory disk.
- 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
option is not set, creating and filling a large malloc-backed memory
disk is a very easy way to panic the system.
- A file specified with -f
file becomes the backing store for
this memory disk.
- 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.
- Bitsink; all writes do nothing, all reads return
- Filename to use for the vnode type memory disk. The
-a and -t
vnode options are implied if not
- List configured devices. If given with
-u, display details about that particular
device. If given with -f
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.
- When printing md(4) device
names, print only the unit number without the
- Size of the memory disk.
Size is the number of 512 byte sectors
unless suffixed with a b,
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
- Sectorsize to use for the memory disk, in bytes.
- See the description of the -y
- 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.
- Set or reset options.
- For vnode backed devices:
IO_SYNC for increased
performance but at the risk of deadlocking the entire kernel.
- Allocate and reserve all needed storage from the start,
rather than as needed.
- Enable clustering on this disk.
- Enable/disable compression features to reduce memory
- 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
- Enable/disable readonly mode.
- 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 be start with “md”
followed by the unit number.
The last form, mdconfig
, is provided for convenience as an
abbreviation of mdconfig
Create a disk with /tmp/boot.flp
storage. The name of the allocated unit will be printed on stdout, such as
Create a 1 gigabyte swap backed memory disk named
Detach and free all resources used by /dev/md3
Show detailed information on current memory disks:
Resize the “
” memory disk to 2
Create a 1 gigabyte swap backed disk, initialize an
file system on it, and mount it on
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
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
the start of the filesystem in the image.
mdconfig -u md1 -f diskimage.img
gnop create -o 512K md1
mount /dev/md1.nop /mnt
utility first appeared in
as a cleaner replacement for the
utility was written by