File access as a block device

1. Loop Device Introduction

In UNIX-like systems, the loop device is a pseudo-device (pseudo-device), or it can be said to be a simulation device. It allows us to access a file like a piece of equipment.
Before use, a loop device must be connected to a file. This combination gives the user an interface to replace the block special file. Therefore, if the file contains a complete file system, the file can be mount like a disk device .
The file format mentioned above, we often see the CD or DVD of the ISO disc image file or floppy disk (hard disk) *.img image file. With this loop mount (loopback mount), these image files can be mount to a directory in the current file system.
Now, by the way, you can understand the meaning of loop: For a first-tier filesystem, it is installed directly on top of our computer's physical device, and for this mounted image file (which also contains a file system), it is built on top of the first file system, so it appears that It is like a file system that loops around the first filesystem, so called loop.

2. Losetup command

Losetup [-E encryption] [-o offset] Loop_device file
Losetup [-d] Loop_device

Description :
This command is used to set up a looping device. The loop device simulates the entire file system by virtualizing the file into a block device, allowing the user to see it as a hard drive, an optical drive or a floppy drive, and hang it as a directory for use.
Above, the options and parameter names in the command format:
-E means encryption
-O Set Data offset
-D Uninstalling the device
Loop_device cycle device name, under Linux such as/dev/loop0,/DEV/LOOP1 and so on.

File name to be associated with the loop device, which is often a disk mirror image file, such as *.img

3. Examples of Use

(1) Create an empty disk image file, create a 1.44M floppy disk here

$ dd If=/dev/zero of=floppy.img bs=512 count=2880

(2) using Losetup to virtual the disk image file into a fast device

$ LOSETUP/DEV/LOOP1 floppy.img

(3) Mounting block equipment

$ mount/dev/loop0/tmp

After three steps above, we can access the disk image file floppy.img through the/tmp directory, like accessing real fast devices.

(4) Uninstalling the loop device

$ umount/tmp

$ losetup-d/DEV/LOOP1

How do I mount an image file?

http://smilejay.com/2012/08/mount-an-image-file/

Or you can use one of the tools that QEMU comes with QEMU-NBD to mount a client.

http://smilejay.com/2012/11/how-to-mount-a-qcow2-image/

File access as a block device