1. Docker image introduction
1.1 Mirror definition
Mirror is a lightweight, executable, independent software package used to package the software operating environment and software developed based on the operating environment. It contains all the content needed to run a certain software, including code, runtime, library, and environment Variables and configuration files.
1.2 UnionFS (Union File System)
Union File System (UnionFS) is a hierarchical, lightweight and high-performance file system. It supports changes to the file system as a submission to overlay layer by layer. At the same time, different directories can be mounted to the same virtual File system.
The Union file system is the basis of Docker mirroring. Mirroring can be inherited through layers. Based on the basic mirroring (without parent mirroring), various specific application mirroring can be made.
1.3 Dcoker image loading principle
The docker image is actually composed of a layered file system, this layered file system UnionFS.
Bootfs (boot file system) mainly includes bootloader and kernel. Bootloader is mainly the bootloader kernel. When Linux starts, it loads the bootfs file system. The bottom layer of the Docker image is bootfs.
This layer is the same as our typical Linux/Unix system, including the boot loader and the kernel.
When the boot load is complete, the entire kernel is in the memory. At this time, the right to use the memory has been transferred from the bootfs to the kernel, and the system will also unload the bootfs at this time.
rootfs (root file system), on top of bootfs. Included are standard directories and files such as /dev, /proc, /bin, /etc in a typical Linux system. Rootfs is a variety of different operating system distributions, such as Ubuntu, Ce