OverlayFS将单个Linux主机上的两个目录合并成一个目录。这些目录被称为层，统一过程被称为联合挂载
OverlayFS底层目录称为lowerdir， 高层目录称为upperdir,合并统一视图称为merged
├───────────────────────────────────│
Container Mount │ FILE 1 │ FILE 2 │ FILE 3 │ FILE 4 │ "merged"
├────↑────────↑────────↑───────↑────│
Container layer │ ↑ │ FILE 2 │ ↑ │ FILE 4 │ "upperdir"
├────↑─────────────────↑────────────│
Image layer │ FILE 1 | FILE 2 | FILE 3 | │ "lowerdir"
├───────────────────────────────────│
- 最下层是
lower层,也是只读/镜像层 upper是容器的读写层,采用了CoW(写时复制)机制,只有对文件进行修改才会将文件拷贝到upper层,之后所有的修改操作都会对upper层的副本进行修改upper并列还有workdir层,它的作用是充当一个中间层的作用,每当对upper层里面的副本进行修改时,会先当到workdir,然后再从workdir移动upper层- 最上层是
mergedir,是一个统一图层,从mergedir可以看到lower,upper,workdir中所有数据的整合,整个容器展现出来的就是mergedir层.
每一个Docker image都是由一系列read-only layer组成的.Image layer的内容都存储在Docker hosts filesystem的/var/lib/docker/overlay2下面
如果在没有拉取任何镜像的前提下,可以发现没有存储任何内容的
root@DESKTOP-UMENNVI:# ls -htral /var/lib/docker/overlay2
总用量 12K
drwx--x--x 15 root root 4.0K 3月 2 17:50 ..
drwx------ 2 root root 4.0K 3月 2 17:50 l ## 这里面都是软连接文件目录的简写标识，这个主要是为了避免mount时候页大小的限制
drwx------ 3 root root 4.0K 3月 2 17:50 .接下来拉取ubuntu:18.04镜像
## 拉取 ubuntu:18.04 镜像
root@DESKTOP-UMENNVI:~# docker pull ubuntu:18.04
18.04: Pulling from library/ubuntu
35c102085707: Downloading
251f5509d51d: Downloading
8e829fe70a46: Downloading
6001e1789921: Downloading
18.04: Pulling from library/ubuntu
423ae2b273f4: Pull complete
de83a2304fa1: Pull complete
f9a83bce3af0: Pull complete
b6b53be908de: Pull complete
Digest: sha256:04d48df82c938587820d7b6006f5071dbbffceb7ca01d2814f81857c631d44df
Status: Downloaded newer image for ubuntu:18.04
docker.io/library/ubuntu:18.04
## 查看 /var/lib/docker/overlay2
root@DESKTOP-UMENNVI:/var/lib/docker/overlay2# tree . -L 3
.
├── 06c3268634199360b720ea7e484497424245bce0ac59ba02843d862eb2d3e975
│ ├── committed
│ ├── diff
│ │ └── var
│ ├── link
│ ├── lower
│ └── work
├── 141562f2c1d9915cdadf58d5d8dbfdbe664ac5645dba4bfbf4665167acef03c3
│ ├── committed
│ ├── diff
│ │ ├── etc
│ │ ├── sbin
│ │ ├── usr
│ │ └── var
│ ├── link
│ ├── lower
│ └── work
├── 1c6bdda320809e237e381ea7bf1936d837f4cb96cfa4fe5ddf2a0878c37d6cf7
│ ├── committed
│ ├── diff
│ │ ├── bin
│ │ ├── boot
│ │ ├── dev
│ │ ├── etc
│ │ ├── home
│ │ ├── lib
│ │ ├── lib64
│ │ ├── media
│ │ ├── mnt
│ │ ├── opt
│ │ ├── proc
│ │ ├── root
│ │ ├── run
│ │ ├── sbin
│ │ ├── srv
│ │ ├── sys
│ │ ├── tmp
│ │ ├── usr
│ │ └── var
│ └── link
├── 758c83af3abbe440d44aa9f8e1fb6feb93a7ea51d5deda031a6cece579f51c04
│ ├── diff
│ │ └── run
│ ├── link
│ ├── lower
│ └── work
└── l
├── 5VTK2ORIHBK33URDA6QHOTNXFW -> ../758c83af3abbe440d44aa9f8e1fb6feb93a7ea51d5deda031a6cece579f51c04/diff
├── CJEDSUSQJVOA4SKGDYKCO6D2BA -> ../1c6bdda320809e237e381ea7bf1936d837f4cb96cfa4fe5ddf2a0878c37d6cf7/diff
├── GRZIGIYAS45PEF36RWVHBM4LGT -> ../06c3268634199360b720ea7e484497424245bce0ac59ba02843d862eb2d3e975/diff
└── U2YACKS2WP5XTR7QY4YSPU6AN4 -> ../141562f2c1d9915cdadf58d5d8dbfdbe664ac5645dba4bfbf4665167acef03c3/diff
41 directories, 10 files-
可以看到
Ubuntu:18.04镜像一共有4个layer,每层的diff即是文件系统在统一挂载时的挂载点 -
l文件夹下都是链接各个layer的软连接 -
查看
1c6bdda320809e237e381ea7bf1936d837f4cb96cfa4fe5ddf2a0878c37d6cf7这个目录,可以看到已经基本系统的雏形了 -
lower文件描述了层序的组织关系(前一个layer依靠后一个layer),比如查看
root@DESKTOP-UMENNVI:/var/lib/docker/overlay2# cat 758c83af3abbe440d44aa9f8e1fb6feb93a7ea51d5deda031a6cece579f51c04/lower
l/U2YACKS2WP5XTR7QY4YSPU6AN4:l/GRZIGIYAS45PEF36RWVHBM4LGT:l/CJEDSUSQJVOA4SKGDYKCO6D2BA接着以Ubuntu:18.04为基础镜像,创建一个名为test-ubuntu的镜像,这个镜像只是在/tmp文件夹中添加了Hello World文件,可以用Dockerfile来实现
FROM ubuntu:18.04
RUN echo "Hello World" > /tmp/newfile执行构建镜像
## 进入到构建镜像文件夹内
root@DESKTOP-UMENNVI:DockerLayer# docker build -t test-ubuntu .
Sending build context to Docker daemon 2.048kB
Step 1/2 : FROM ubuntu:18.04
---> 72300a873c2c
Step 2/2 : RUN echo "Hello World" > /tmp/newfile
---> Running in 0644707c9a4c
Removing intermediate container 0644707c9a4c
---> 8fe0d75009f9
Successfully built 8fe0d75009f9
Successfully tagged test-ubuntu:latest然后执行 docker images 查看现有的镜像,可以清楚查看到test-ubuntu使用了哪些image layer
root@DESKTOP-UMENNVI:# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
test-ubuntu latest 8fe0d75009f9 27 minutes ago 64.2MB
ubuntu 18.04 72300a873c2c 9 days ago 64.2MB
## 查看镜像构建layer层
root@DESKTOP-UMENNVI:# docker history test-ubuntu
IMAGE CREATED CREATED BY SIZE COMMENT
8fe0d75009f9 27 minutes ago /bin/sh -c echo "Hello World" > /tmp/newfile 12B
72300a873c2c 9 days ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0B
<missing> 9 days ago /bin/sh -c mkdir -p /run/systemd && echo 'do… 7B
<missing> 9 days ago /bin/sh -c set -xe && echo '#!/bin/sh' > /… 745B
<missing> 9 days ago /bin/sh -c [ -z "$(apt-get indextargets)" ] 987kB
<missing> 9 days ago /bin/sh -c #(nop) ADD file:91a750fb184711fde… 63.2MB 从输出中可以看到8fe0d75009f9 image layer 位于最上层,只有12B大小,由以下命令创建
/bin/sh -c echo "Hello World" > /tmp/newfile所以我们构建的镜像test-ubuntu只占用了12B的磁盘空间, 而最下面的四层image layer则是构成ubuntu:18.04镜像的4个image layer.
<missing>标记的layer,是因为在Docker v1.10版本 之前,每次都会创建一个新的图层作为commit操作的结果,Docker也会创建一个相应的图像,该图像由随机生成的256位UUID，通常称为图像ID(在用户界面中显示为12位十六进制字符串或长64位十六进制字符串)
Docker将图层内容存储在目录中,并且可以给每个图层添加镜像名称(比如mysql:5.1.1.1),但很多图层其实是缓存(中间层)并不算是最终的镜像
所以从Docker v1.10开始,一个镜像可以包含多个图层,并且显示父级镜像的sha256值,其他图层则用<missing>代替
------------------------------
接下来查看test-ubuntu存储信息
root@DESKTOP-UMENNVI:/var/lib/docker/overlay2# docker inspect test-ubuntu
[
{
"Id": "sha256:8fe0d75009f9b670a0de0a42e47e54e924af8d053b6e06e3ec8f29879f320e9e",
"RepoTags": [
"test-ubuntu:latest"
],
"RepoDigests": [],
"Parent": "sha256:72300a873c2ca11c70d0c8642177ce76ff69ae04d61a5813ef58d40ff66e3e7c",
"Comment": "",
"Created": "2020-03-02T14:23:40.7871895Z",
"Container": "0644707c9a4c2377e848d9766caea05a97b29499df7c89ac67a6b170694895ef",
"ContainerConfig": {
"Hostname": "",
"Domainname": "",
"User": "",
"AttachStdin": false,
"AttachStdout": false,
"AttachStderr": false,
"Tty": false,
"OpenStdin": false,
"StdinOnce": false,
"Env": [
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
],
"Cmd": [
"/bin/sh",
"-c",
"echo \"Hello World\" > /tmp/newfile"
],
"Image": "sha256:72300a873c2ca11c70d0c8642177ce76ff69ae04d61a5813ef58d40ff66e3e7c",
"Volumes": null,
"WorkingDir": "",
"Entrypoint": null,
"OnBuild": null,
"Labels": null
},
"DockerVersion": "19.03.4",
"Author": "",
"Config": {
"Hostname": "",
"Domainname": "",
"User": "",
"AttachStdin": false,
"AttachStdout": false,
"AttachStderr": false,
"Tty": false,
"OpenStdin": false,
"StdinOnce": false,
"Env": [
"PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
],
"Cmd": [
"/bin/bash"
],
"ArgsEscaped": true,
"Image": "sha256:72300a873c2ca11c70d0c8642177ce76ff69ae04d61a5813ef58d40ff66e3e7c",
"Volumes": null,
"WorkingDir": "",
"Entrypoint": null,
"OnBuild": null,
"Labels": null
},
"Architecture": "amd64",
"Os": "linux",
"Size": 64206129,
"VirtualSize": 64206129,
"GraphDriver": {
"Data": {
"LowerDir": "/var/lib/docker/overlay2/758c83af3abbe440d44aa9f8e1fb6feb93a7ea51d5deda031a6cece579f51c04/diff:/var/lib/docker/overlay2/141562f2c1d9915cdadf58d5d8dbfdbe664ac5645dba4bfbf4665167acef03c3/diff:/var/lib/docker/overlay2/06c3268634199360b720ea7e484497424245bce0ac59ba02843d862eb2d3e975/diff:/var/lib/docker/overlay2/1c6bdda320809e237e381ea7bf1936d837f4cb96cfa4fe5ddf2a0878c37d6cf7/diff",
"MergedDir": "/var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6/merged",
"UpperDir": "/var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6/diff",
"WorkDir": "/var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6/work"
},
"Name": "overlay2"
},
"RootFS": {
"Type": "layers",
"Layers": [
"sha256:cc4590d6a7187ce8879dd8ea931ffaa18bc52a1c1df702c9d538b2f0c927709d",
"sha256:8c98131d2d1d1c8339d268265005394de6cafe887020dcca9ba9d9d07a56280c",
"sha256:03c9b9f537a4ae66d7ae7a4361e7f36e6755380107eadff3fbc11cd604c6c9b9",
"sha256:1852b2300972ff9f68fd3d34f1f112df3e35757d91dcd40cc8b379bbf2be62d5",
"sha256:8904c7689676b958102eff5a32550ed1c31348a2e99fab5233b5d5d11f4db187"
]
},
"Metadata": {
"LastTagTime": "2020-03-02T22:23:40.8005173+08:00"
}
}
]
从GraphDriver.Data可以看出我们新创建的图层在/var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6
## 进入到 /var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6 内
oot@DESKTOP-UMENNVI:~# cd /var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6
## 查看目录
root@DESKTOP-UMENNVI:/var/lib/docker/overlay2/1c9b3562edc8d89cd05813a4d2793e1de2694dc7f4f2f7e18684acf72f3726e6# tree .
.
├── diff
│ └── tmp
│ └── newfile
├── link
├── lower
└── work
3 directories, 3 files
在diff/ 层下面多了一个tmp/newfile文件,这样我们在创建容器的时候,会根据LowerDir -> UpperDir/WorkDir -> MergedDir层最终在MergedDir层展示出来