17.13. Network, Memory, and File-Backed File Systems

17.13.1. File-Backed File System

The utility mdconfig(8) is used to configure and enable memory disks, md(4), under FreeBSD. 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
# bsdlabel -w md0 auto
# newfs md0a
/dev/md0a: 5.0MB (10224 sectors) block size 16384, fragment size 2048
        using 4 cylinder groups of 1.25MB, 80 blks, 192 inodes.
super-block backups (for fsck -b #) at:
 160, 2720, 5280, 7840
# mount /dev/md0a /mnt
# df /mnt
Filesystem 1K-blocks Used Avail Capacity  Mounted on
/dev/md0a       4710    4  4330     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 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        4718    4  4338     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.

17.13.2. Memory-Based File System

For a memory-based file system the “swap backing” should normally be used. Using swap backing does not mean that the memory disk will be swapped out to disk by default, but merely that the memory disk will be allocated from a memory pool which can be swapped out to disk if needed. It is also possible to create memory-based disk which are malloc(9) backed, but using malloc backed memory disks, especially large ones, can result in a system panic if the kernel runs out of memory.

# mdconfig -a -t swap -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, 192 inodes.
        with soft updates
super-block backups (for fsck -b #) at:
 160, 2752, 5344, 7936
# mount /dev/md1 /mnt
# df /mnt
Filesystem 1K-blocks Used Avail Capacity  Mounted on
/dev/md1        4718    4  4338     0%    /mnt

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

17.13.3. 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.