Detailed explanation of loop device in Linux

What is a loop device?

Loop device is a pseudo device that uses files to simulate block device technology. By emulating a block device, the file can be used like a disk or optical disk. The loop device must be connected to a file before use. This combination provides an alternative to block special files. If the file contains a complete file system, it can be mounted like a disk device. This device is called a loop device and is considered from the file system level. After the image file is mounted, it itself also contains a file system. Mounting it through the loop device is equivalent to adding another layer of file system on top of the file system. Therefore, it is called loop device.

2. Use of loop device

Generally, there are 8 loop devices in Linux, usually /dev/loop0~loop7. You can use losetup -a to view all loop devices. If the command does not output, it means that all loop devices are not occupied. You can You can create your own loop device by following the steps below.

1) Create a file

dd if=/dev/zero of=/var/loop.img bs=1M count=10240

2) Use losetup to convert the file into a block device

losetup /dev/loop0 /var/loop.img

3) View the block device just created through lsblk

lsblk |grep loop0
losetup -a

4) Of course, you can also format this block device and create other file systems, and then mount it to a certain directory. This is a bit redundant. Most people don’t do this.

5) To delete this loop device, you can execute the following command

losetup -d /dev/loop0

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