Docker的overlay2学习

1. docker配置overlay2

# vi /etc/docker/daemon.json
{
    "storage-driver": "overlay2",
    "storage-opts": ["overlay2.override_kernel_check=true"],
}

2. overlay2目录结构

[root@test01 ~]# ll /var/lib/docker/overlay2/
total 12
drwx------ 5 root root     69 Sep 15 14:40 028ddcc599a0efcd5e6cb26c75215922e3252067796f9e7b6473aa49a5ba48f8
drwx------ 5 root root     69 Sep 15 14:40 028ddcc599a0efcd5e6cb26c75215922e3252067796f9e7b6473aa49a5ba48f8-init
drwx------ 3 root root     20 Sep 14 19:21 053df919d2fbfccf983e4839498089433447d23da1b793877e3f618386fe8f69
drwx------ 5 root root     69 Sep 14 10:35 053df919d2fbfccf983e4839498089433447d23da1b793877e3f618386fe8f69-init
drwx------ 4 root root     55 Sep  6 23:38 06a3b274a3ea7b40ec598fd9161c1eef82740b911cc7ccec47be44691d5ca204
drwx------ 5 root root     69 Sep 13 21:35 122ac9fffa7c0bd4ef51f8502005ee5ebec0414d3edd531a7bb4e52dd666cbbb
drwx------ 4 root root     55 Jun 17 11:52 15b8646cc191794f709252e6ff8c991850a62f2cbe5346ca3af363f4a517ca50
drwx------ 2 root root   8192 Sep 15 16:29 l

l目录存放了短软链接指向了CacheID目录下的diff目录，这是为了挂载命令不会超出长度

[root@test01 overlay2]# ll l/
total 0
lrwxrwxrwx 1 root root 77 Sep 14 10:35 26F6RXAGEGTYRGSYS2AR5N3CRA -> ../b3b028a6dfe8efdaea3c94c873da64b8ad596ec8e03a5ab5880c4ddf40081385-init/diff
lrwxrwxrwx 1 root root 72 Sep 13 21:39 36YZLFLIWX2FV2OGFBNTLN6WTF -> ../49e66854c03251cc3884e49ffb81272d0dadb5bc6a554348debccfd7836d8f2d/diff
lrwxrwxrwx 1 root root 72 Sep  6 23:36 36ZOKMSGUFLPHGNMOO4FYUP4ZZ -> ../cb4ed32920576dd6b7f297cab5c01c4c8ab06e00a903467e88b9b847e3da2dc3/diff
...

最底层只有diff和link文件，link文件内容是l目录里面的缩写链接名，实际就是对应diff目录，diff目录实际存放这一层的文件内容

[root@test01 overlay2]# ll f51afd73a6c375929372459583ca58ad4c35f6696562e0cd9764d86c8a0d5abf
total 8
drwxr-xr-x 8 root root 73 May 25 08:24 diff
-rw-r--r-- 1 root root 26 May 25 08:24 link
-rw-r--r-- 1 root root 57 May 25 08:24 lower
drwx------ 2 root root  6 May 25 08:24 work

[root@test01 overlay2]# ll f51afd73a6c375929372459583ca58ad4c35f6696562e0cd9764d86c8a0d5abf/diff/
total 0
drwxr-xr-x 3 root root 45 May 16 10:11 etc
drwxr-xr-x 4 root root 41 May 16 10:11 lib
drwx------ 2 root root 24 May 16 10:11 root
drwxrwxrwt 2 root root  6 May 16 10:11 tmp
drwxr-xr-x 7 root root 66 May 14 22:50 usr
drwxr-xr-x 5 root root 41 May 14 22:50 var

高位层有以上目录，如果有多个低位层则会有lower文件，对应文件内容：

[root@test01 overlay2]# cat f51afd73a6c375929372459583ca58ad4c35f6696562e0cd9764d86c8a0d5abf/lower
l/TEU77L2PYR6NJ72JLNXTCVELCH:l/RMPLZG2O42VZCADGWLYKDR6STQ

启动容器后的目录挂载情况：

[root@test01 overlay2]# mount | grep overlay2
/dev/mapper/centos-root on /var/lib/docker/overlay2 type xfs (rw,relatime,attr2,inode64,noquota)
overlay on /var/lib/docker/overlay2/1eeac41cfc97cfc9b3a25581870b130ec2c7c64b679b9002be3bfbe1123907d4/merged type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/YGDXGAURB7D26RG7ZCQ45PCD57:/var/lib/docker/overlay2/l/I3OKPZLICEBMHWRLPFSJOW64MY,upperdir=/var/lib/docker/overlay2/1eeac41cfc97cfc9b3a25581870b130ec2c7c64b679b9002be3bfbe1123907d4/diff,workdir=/var/lib/docker/overlay2/1eeac41cfc97cfc9b3a25581870b130ec2c7c64b679b9002be3bfbe1123907d4/work)

overlay2分upperdir和lowerdir层：
读文件的时候，文件不在upperdir则从lowerdir读，如果写的文件不在uppderdir在lowerdir，则从lowerdir里面copy到upperdir，不管文件多大，copy完再写，删除或者重命名镜像层的文件都只是在容器层生成whiteout文件标志。

overlay2支持多个容器访问相同文件时公用page cache，在copy_up的时候overlay2比aufs更快，因为aufs的层级多搜索会有延迟，而overlay2会有缓存机制。优化方面可以用ssd，然后频繁的io操作，可以通过挂载volumes来做，绕过存储驱动，而且可以多个容器共享数据，持久化数据。

3. overlay和overlay2的区别
   overlay: 只挂载一层,其他层通过最高层通过硬连接形式共享(增加了磁盘inode的负担)
   overlay2: 逐层挂载