How to create and delete loopback devices by losetup command?

Environment

• Red Hat Enterprise Linux 7
• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 9

Issue

• How can I create and delete loopback devices by losetup command?
• Use losetup to create a loop device for an existing FS/image.

Resolution

• Creating a loopback device:

  1. Preparing an appropriately sized file for the loopback device:

     For example:

     # dd if=/dev/zero of=/tmp/test.img bs=1M count=100
     

  2. Attaching a device to the file:

     Specifying a loop device directly or using the -f option to automatically assign an unused device

     # losetup /dev/loop0 /tmp/test.img
     

     or

     # losetup -f /tmp/test.img
     

  3. Checking the created loop device, and creating/mounting an FS:

     You can check the status of all loop devices with the -a option:

     # losetup -a
     /dev/loop0: [64770]:134591359 (/tmp/test.img)
     

     To check a specific device by reverse lookup from a file, use the -j option:

     # losetup -j /tmp/test.img
     /dev/loop0: [64770]:134591359 (/tmp/test.img)
     

     Format and mount with a specific file system:

     # mkfs.xfs /tmp/test.img
     # mount /dev/loop0 /mnt -v
     

• Deleting a loopback device:

  If the loop device is mounted, unmount it, delete the loop device, and then delete the corresponding file:

  # umount /dev/loop0
  # losetup -d /dev/loop0
  # rm -f /tmp/test.img
  

• For any existing FS image, or FS metadata where signature is detected, the FS could be mounted using losetup directly as a loop device:

  # losetup -f </path/to/image/having/valid/signature>
  

  Note: The image layout must be inspected first for identifying what type of image it is.

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