Docker联合文件系统

摘录几篇比较好的参考文档：

1.Docker 联合文件系统(Union Filesystem): https://www.jianshu.com/p/5ec3d4dbf580

2.Visualizing Docker Containers and Images： http://merrigrove.blogspot.com/2015/10/visualizing-docker-containers-and-images.html

3.About storage drivers： https://docs.docker.com/storage/storagedriver/

摘录自参考文档一：

假设Dockerfile 内容如下
FROM ubuntu:14.04
ADD run.sh /
VOLUME /data
CMD ["./run.sh"]
联合文件系统对应的层次结构如下图所示

摘录自参考文档二：

镜像（Images）：

The first visual I present is that of an image, shown below with two different visuals.  It is defined as the "union view" of a stack of read-only layers.

一层一层的只读层，代表本层和下面层的diff，若本层有文件系统的改动，则Read Layer包含具体改动后的文件，若没有文件系统的改动（比如仅仅只是增加了环境变量等），会记录到Meta Json中。

镜像层是存储在主机文件系统中的，针对aufs这种类型的文件系统驱动，可以去查看/var/lib/docker/aufs下的目录结构

# sudo tree -L 1 /var/lib/docker/

/var/lib/docker/

├── aufs

├── containers

├── graph

├── init

├── linkgraph.db

├── repositories-aufs

├── tmp

├── trust

└── volumes

Finally, to tie up some loose ends, we should define an image layer. The below image shows an image layer and makes us realize that a layer is not just the changes to the file system.

 

The metadata is additional information about the layer that allows docker to capture runtime and build-time information, but also hierarchical information on a layer's parent. Both read and read-write layers contain this metadata.

 容器（Container）：

A container is defined as a "union view" of a stack of layers the top of which is a read-write layer.

运行中的容器（Running Container）：

 A running container is defined as a read-write "union view" and the the isolated process-space and processes within.  The below visual shows the read-write container surrounded by this process-space.