系列文章：
Docker镜像存储-overlayfs
docker背景知识3 镜像分层(overlay2)
Docker存储驱动之–overlay2

1. docker支持哪些分层文件系统

当前docker支持的分层文件系统参见下表

Linux发行版本	推荐使用的存储系统	可替代的其他技术
Ubuntu	overlay2 or aufs	overlay, devicemapper, zfs, vfs
Debian	overlay2, aufs or devicemapper	overlay, vfs
CentOS	overlay2	overlay, devicemapper, zfs, vfs
Fedora	overlay2	overlay, devicemapper, zfs, vfs

linux overlay filesystem

在overlayFS，中底层目录叫lowerdir，顶层目录叫upperdir，对外统一提供服务的是merged，当修改下层文件时，会触发写时复制

2. 探索overlayFS

❯ cd /root/test/test
❯ mkdir layer1 layer2 upperlayer workdir mountedfs
❯ echo "content for file1.txt in layer1" > layer1/file1.txt
❯ echo "content for file2.txt in layer1" > layer1/file2.txt
❯ echo "content for file1.txt in layer2" > layer2/file1.txt
❯ echo "content for file3.txt in layer2" > layer2/file3.txt

此时的目录结构：

[root@localhost test]# pwd
/root/test/test
[root@localhost test]# tree
.
├── layer1
│   ├── file1.txt
│   └── file2.txt
├── layer2
│   ├── file1.txt
│   └── file3.txt
├── mountedfs
├── upperlayer
└── workdir

挂载生成一个overlayFS

sudo mount -t overlay -o lowerdir=layer1:layer2,upperdir=upperlayer,workdir=workdir overlay mountedfs

此时的目录结构：

[root@localhost test]# pwd
/root/test/test
[root@localhost test]# tree
.
├── layer1
│   ├── file1.txt
│   └── file2.txt
├── layer2
│   ├── file1.txt
│   └── file3.txt
├── mountedfs
│   ├── file1.txt
│   ├── file2.txt
│   └── file3.txt
├── upperlayer
└── workdir└── work

挂载后查看所有目录, mountedfs中有三个文件，查看文件file1发现使用的是上层layer1中的

❯ ls -l *       '类似tree，显示全部文件列表，包含子目录，只不过是平铺的'
layer1:
total 8
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:41 file1.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file2.txtlayer2:
total 8
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:41 file1.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file3.txt//merged层，2个同名的file1.txt过滤为1个
mountedfs:
total 12
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:41 file1.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file2.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file3.txt//upperdir目前为空，此层为复制层，没有产生复制文件
upperlayer:
total 0workdir:
total 0
d--------- 2 root root 6 Apr 13 00:39 work//mountedfs/file1.txt 发现使用的是底层layer1中的，为何使用的是layer1中的呢？因为lowerdir=layer1:layer2中layer1在layer2前，保留先出现的
❯ cat mountedfs/file1.txt    
content for file1.txt in layer1

修改mountedfs/file1.txt

❯ echo newfile in mountedfs > mountedfs/file1.txt

再次查看所有目录，发现upperlayer中多一个file1.txt，其中内容为newfile，layer1还是原值

❯ ls -l *
layer1:
total 8
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:41 file1.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file2.txtlayer2:
total 8
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:41 file1.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file3.txtmountedfs:
total 12
-rw-r--r-- 1 wsl zdevel 21 Apr 13 00:44 file1.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file2.txt
-rw-r--r-- 1 wsl zdevel 32 Apr 13 00:37 file3.txt//复制层，产生一个文件
upperlayer:
total 4
-rw-r--r-- 1 wsl zdevel 21 Apr 13 00:44 file1.txtworkdir:
total 0
d--------- 2 root root 6 Apr 13 00:44 work//查看变化，生效了
❯ cat mountedfs/file1.txt
newfile in mountedfs//复制层的内容就是 mountedfs/file1.txt
❯ cat upperlayer/file1.txt
newfile in mountedfs//layer1中的原始文件没变，说明修改下层文件时，会触发写时复制
❯ cat layer1/file1.txt
content for file1.txt in layer1

3. docker中的使用

3.1 在docker中启用overlayfs

查看当前版本的docker采用的分层类型：

❯ cat /etc/docker/daemon.json
{"storage-driver": "overlay2",      '说明分层类型是overlay2'"insecure-registries": ["0.0.0.0/0"],"registry-mirrors": ["https://f0z3hvz3.mirror.aliyuncs.com"]
}

3.2 查看容器的overlay挂载情况

❯ sudo docker inspect xxxDockerId
...
"GraphDriver": {"Data": {"LowerDir": "/var/lib/docker/overlay2/fd7a593c88623dbbeb12a897d5d293751d11a17a4f720ddfe83a1f381271686c-init/diff:/var/lib/docker/overlay2/064fae2fb1a163e591db5c9d6f3829b0beee31c079687fff523a52a712fea746/diff:/var/lib/dock
er/overlay2/254f7f0509dfae6f4a11c1f11149ce1367ccc3415f5727ce2e75d5891f3de68e/diff","MergedDir": "/var/lib/docker/overlay2/fd7a593c88623dbbeb12a897d5d293751d11a17a4f720ddfe83a1f381271686c/merged","UpperDir": "/var/lib/docker/overlay2/fd7a593c88623dbbeb12a897d5d293751d11a17a4f720ddfe83a1f381271686c/diff","WorkDir": "/var/lib/docker/overlay2/fd7a593c88623dbbeb12a897d5d293751d11a17a4f720ddfe83a1f381271686c/work"},"Name": "overlay2"
},
...