12.11 Network, Memory, and File-Backed File Systems

Reorganized and enhanced by Marc Fonvieille.

Aside from the disks you physically insert into your computer: floppies, CDs, hard drives, and so forth; other forms of disks are understood by DragonFly - the virtual disks.

These include network file systems such as the Network File System, memory-based file systems and file-backed file systems.

12.11.1 File-Backed File System

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:

Example 12-4. Using vnconfig to Mount an Existing File System Image

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

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

Example 12-5. Creating a New File-Backed Disk with vnconfig

# 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:
# mount /dev/vn0c /mnt
# df /mnt
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/vn0c        4927        1     4532     0%    /mnt

12.11.2 Memory-Based File System

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

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

Example 12-6. md Memory Disk

# 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.

12.11.3 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.

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

# vnconfig -u vn4

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