Making use of loop devices on Linux

This post is about the loop devices on Linux.

The loop devices are a possibility under Linux to mount regular files. It is often used to mount an iso image, but it can also be used to play with Raid, LVM, or a file system.

To use the loop devices it is mandatory to load the loop module: ”modprobe loop”. This will create the loop devices /dev/loop0, /dev/loop1, etc.

Remark: work inside a tmpfs like /dev/shm to gain speed when running tests.

Creating a file-system out of a file, plus being able to mount it, works as follow:
  1. Create a file of a hundred mega-bytes:
    • dd bs=1M count=100 if=/dev/zero of=output
  2. Format the file:
    • mkfs.ext2 output
  3. Mount the file with a loop device:
    • mkdir loop-mount
    • mount -o loop output loop-mount
    • When using the option ”-o loop” the first available loop device is taken. It is possible to select the loop device to use by giving the option ”-o loop=/dev/loop0”.
To check that the device is perfectly mounted, run ”mount”. It displays a line like that one:
/dev/shm/output on /dev/shm/loop-mount type ext2 (rw,loop=/dev/loop0)
It is possible to create files and directories within loop-mount, then unmount the device and remount it back.

Using loop devices is very convenient when training with Raid for instance. To create usable devices for a Raid, it works as for a file-system, except that instead of mounting the file as a loop device they must be setup with the tool ”losetup” that is part of the Linux utilities. In fact, it is not possible to prepare devices for a Raid that are already mounted, and losetup makes a loop device in /dev/ ready to be mounted in a way that it is possible to run ”mount /dev/loop0 mount-point” instead of ”mount -o loop file mount-point”.

Preparing a Raid5 works as follow:
  1. Create 3 files:
    • dd bs=1M count=100 if=/dev/zero of=0
    • dd bs=1M count=100 if=/dev/zero of=1
    • dd bs=1M count=100 if=/dev/zero of=2
  2. Format the files:
    • mkfs.ext2 0
    • mkfs.ext2 1
    • mkfs.ext2 2
  3. Setup loop devices:
    • losetup /dev/loop0 0
    • losetup /dev/loop1 1
    • losetup /dev/loop2 2
  4. Prepare the Raid5 device:
    • mdadm --create --verbose /dev/md0 --level=5 --raid-devices=3 /dev/loop0 /dev/loop1 /dev/loop2
  5. Mount the Raid5 device:
    • mount /dev/md0 mount-point