MDMFS(8) MidnightBSD System Manager’s Manual MDMFS(8)


mdmfs, mount_mfs — configure and mount an in-memory file system using the md(4) driver


mdmfs [−DLlMNSUX] [−a maxcontig] [−b block-size] [−c cylinders] [−d rotdelay] [−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
mdmfs −C
[−lNU] [−a maxcontig] [−b block-size] [−c cylinders] [−d rotdelay] [−e maxbpg] [−F file] [−f frag-size] [−i bytes] [−m percent-free] [−n rotational-positions] [−O optimization] [−o mount-options] [−s size] [−v version] md-device mount-point


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 using newfs(8), and mounts it using mount(8). 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 specified 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.


Enable full compatibility mode with mount_mfs(8). See the COMPATIBILITY section for more information.

−c cylinders

The number of cylinders per cylinder group in the file system.


If not using auto-unit, do not run mdconfig(8) to try to detach the unit before attaching it.

−d rotdelay

Specify the minimum time in milliseconds required to initiate another disk transfer on the same cylinder. Modern disks with read/write-behind achieve higher performance without this feature, so it is best to leave it at 0 milliseconds.

−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.


Enable multilabel MAC on the new file system.


Show the output of the helper programs. By default, it is sent to /dev/null.


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.


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 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’’.


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).


Enable soft-updates on the file system. This is the default, even in compatibility mode, 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.


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.


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


The mdmfs utility, while designed to be fully compatible with mount_mfs(8), can be useful by itself. Since mount_mfs(8) had some silly defaults, a ‘‘full compatibility’’ mode is provided for the case where bug-to-bug compatibility is desired.

Full compatibility is enabled with the −C flag, or by starting mdmfs with the name mount_mfs or mfs (as returned by getprogname(3)). In this mode, only the options which would be accepted by mount_mfs(8) are valid. Furthermore, the following behavior, as done by mount_mfs(8), is duplicated:

The file mode of mount-point is set to 01777 as if −p 1777 was given on the command line.


md(4), fstab(5), mdconfig(8), mount(8), newfs(8)


Dima Dorfman

MidnightBSD 0.3 February 26, 2004 MidnightBSD 0.3