Loopback Devices

• A loopback device enables you to simulate a block device using an ordinary disk file.
• Imagine a disk drive device for which data is written to and read from a file named disk-image rather than to and from the tracks and sectors of an actual physical disk drive or disk partition.
• A loopback device enables you to use a file in this manner.
• Loopback devices are named /dev/loop0, /dev/loop1, and so on.
• A loopback device can be used in the same way as any other block device. In particular, you can construct a file system on the device and then mount that file system as you would mount the file system on an ordinary disk or partition (virtual file system).
• To construct a virtual file system and mount it with a loopback device, follow these steps:
  1. Create an empty file to hold the virtual file system. To construct a 10MB file named disk-image, invoke the following:

     $ dd if=/dev/zero of=/yourdirectory/disk-image count=20480
     $ ls -l /yourdirectory/disk-image
     

  2. The file that you've just created is filled with 0 bytes. Before you mount it, you must construct a file system. This sets up the various control structures needed to organize and store files, and builds the root directory.

     $ mke2fs -q /yourdirectory/disk-image
     

  3. Mount the file system using a loopback device.

     $ mkdir /yourdirectory/virtual-fs
     $ mount -o loop=/dev/loop0 /yourdirectory/disk-image
       /yourdirectory/virtual-fs
     $ df -h /yourdirectory/virtual-fs
     $ cd /yourdirectory/virtual-fs
     $ echo "Hello, world!" > test.txt
     $ ls -l
     $ cat test.txt
     $ cd /yourdirectory
     $ umount /yourdirectory/virtual-fs