16.12 Network, Memory, and File-Backed File Systems

16.12.1 File-Backed File System under FreeBSD 4.X

The utility vnconfig(8) configures and enables vnode pseudo-disk devices. A vnode is a representation of a file, and is the focus of file activity. This means that vnconfig(8) uses files to create and operate a file system. One possible use is the mounting of floppy or CD images kept in files.

To use vnconfig(8), you need vn(4) support in your kernel configuration file:

pseudo-device vn

To mount an existing file system image:

# vnconfig vn0 diskimage
# mount /dev/vn0c /mnt

To create a new file system image with vnconfig(8):

# dd if=/dev/zero of=newimage bs=1k count=5k
5120+0 records in
5120+0 records out
# vnconfig -s labels -c vn0 newimage
# disklabel -r -w vn0 auto
# newfs vn0c
Warning: 2048 sector(s) in last cylinder unallocated
/dev/vn0c:     10240 sectors in 3 cylinders of 1 tracks, 4096 sectors
        5.0MB in 1 cyl groups (16 c/g, 32.00MB/g, 1280 i/g)
super-block backups (for fsck -b #) at:
 32
# mount /dev/vn0c /mnt
# df /mnt
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/vn0c        4927        1     4532     0%    /mnt

16.12.2 File-Backed File System under FreeBSD 5.X

The utility mdconfig(8) is used to configure and enable memory disks, md(4), under FreeBSD 5.X. To use mdconfig(8), you have to load md(4) module or to add the support in your kernel configuration file:

device md

The mdconfig(8) command supports three kinds of memory backed virtual disks: memory disks allocated with malloc(9), memory disks using a file or swap space as backing. One possible use is the mounting of floppy or CD images kept in files.

To mount an existing file system image:

# mdconfig -a -t vnode -f diskimage -u 0
# mount /dev/md0 /mnt

To create a new file system image with mdconfig(8):

# dd if=/dev/zero of=newimage bs=1k count=5k
5120+0 records in
5120+0 records out
# mdconfig -a -t vnode -f newimage -u 0
# disklabel -r -w md0 auto
# newfs md0c
/dev/md0c: 5.0MB (10240 sectors) block size 16384, fragment size 2048
    using 4 cylinder groups of 1.27MB, 81 blks, 256 inodes.
super-block backups (for fsck -b #) at:
 32, 2624, 5216, 7808
# mount /dev/md0c /mnt
# df /mnt
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/md0c        4846        2     4458     0%    /mnt

If you do not specify the unit number with the -u option, mdconfig(8) will use the md(4) automatic allocation to select an unused device. The name of the allocated unit will be output on stdout like md4. For more details about mdconfig(8), please refer to the manual page.

The utility mdconfig(8) is very useful, however it asks many command lines to create a file-backed file system. FreeBSD 5.0 also comes with a tool called mdmfs(8), this program configures a md(4) disk using mdconfig(8), puts a UFS file system on it using newfs(8), and mounts it using mount(8). For example, if you want to create and mount the same file system image as above, simply type the following:

# dd if=/dev/zero of=newimage bs=1k count=5k
5120+0 records in
5120+0 records out
# mdmfs -F newimage -s 5m md0 /mnt
# df /mnt
Filesystem 1K-blocks Used Avail Capacity  Mounted on
/dev/md0        4846    2  4458     0%    /mnt

If you use the option md without unit number, mdmfs(8) will use md(4) auto-unit feature to automatically select an unused device. For more details about mdmfs(8), please refer to the manual page.

16.12.3 Memory-Based File System under FreeBSD 4.X

The md(4) driver is a simple, efficient means to create memory file systems under FreeBSD 4.X. malloc(9) is used to allocate the memory.

Simply take a file system you have prepared with, for example, vnconfig(8), and:

# dd if=newimage of=/dev/md0
5120+0 records in
5120+0 records out
# mount /dev/md0c /mnt
# df /mnt
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/md0c        4927        1     4532     0%    /mnt

For more details, please refer to md(4) manual page.

16.12.4 Memory-Based File System under FreeBSD 5.X

The same tools are used for memory-based and file-backed file systems: mdconfig(8) or mdmfs(8). The storage for memory-based file system is allocated with malloc(9).

# mdconfig -a -t malloc -s 5m -u 1
# newfs -U md1
/dev/md1: 5.0MB (10240 sectors) block size 16384, fragment size 2048
    using 4 cylinder groups of 1.27MB, 81 blks, 256 inodes.
    with soft updates
super-block backups (for fsck -b #) at:
 32, 2624, 5216, 7808
# mount /dev/md1 /mnt
# df /mnt
Filesystem 1K-blocks Used Avail Capacity  Mounted on
/dev/md1        4846    2  4458     0%    /mnt

# mdmfs -M -s 5m md2 /mnt
# df /mnt
Filesystem 1K-blocks Used Avail Capacity  Mounted on
/dev/md2        4846    2  4458     0%    /mnt

Instead of using a malloc(9) backed file system, it is possible to use swap, for that just replace malloc with swap in the command line of mdconfig(8). The mdmfs(8) utility by default (without -M) creates a swap-based disk. For more details, please refer to mdconfig(8) and mdmfs(8) manual pages.

16.12.5 Detaching a Memory Disk from the System

When a memory-based or file-based file system is not used, you should release all resources to the system. The first thing to do is to unmount the file system, then use mdconfig(8) to detach the disk from the system and release the resources.

For example to detach and free all resources used by /dev/md4:

# mdconfig -d -u 4

It is possible to list information about configured md(4) devices in using the command mdconfig -l.

For FreeBSD 4.X, vnconfig(8) is used to detach the device. For example to detach and free all resources used by /dev/vn4:

# vnconfig -u vn4