docker 存储驱动之 overlay2

overlay2

overlay2原生支持128层，这提供docker build和docker commit更好的性能支持 
在执行完docker pull ubuntu后，可以看到

1 $ ls -l /var/lib/docker/overlay2
2 
3 total 24
4 drwx------ 5 root root 4096 Jun 20 07:36 223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7
5 drwx------ 3 root root 4096 Jun 20 07:36 3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b
6 drwx------ 5 root root 4096 Jun 20 07:36 4e9fa83caff3e8f4cc83693fa407a4a9fac9573deaf481506c102d484dd1e6a1
7 drwx------ 5 root root 4096 Jun 20 07:36 e8876a226237217ec61c4baf238a32992291d059fdac95ed6303bdff3f59cff5
8 drwx------ 5 root root 4096 Jun 20 07:36 eca1e4e1694283e001f200a667bb3cb40853cf2d1b12c29feda7422fed78afed
9 drwx------ 2 root root 4096 Jun 20 07:36 l

这个 l 目录是新加的，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

1 $ ls -l /var/lib/docker/overlay2/l
2 
3 total 20
4 lrwxrwxrwx 1 root root 72 Jun 20 07:36 6Y5IM2XC7TSNIJZZFLJCS6I4I4 -> ../3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/diff
5 lrwxrwxrwx 1 root root 72 Jun 20 07:36 B3WWEFKBG3PLLV737KZFIASSW7 -> ../4e9fa83caff3e8f4cc83693fa407a4a9fac9573deaf481506c102d484dd1e6a1/diff
6 lrwxrwxrwx 1 root root 72 Jun 20 07:36 JEYMODZYFCZFYSDABYXD5MF6YO -> ../eca1e4e1694283e001f200a667bb3cb40853cf2d1b12c29feda7422fed78afed/diff
7 lrwxrwxrwx 1 root root 72 Jun 20 07:36 NFYKDW6APBCCUCTOUSYDH4DXAT -> ../223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7/diff
8 lrwxrwxrwx 1 root root 72 Jun 20 07:36 UL2MW33MSE3Q5VYIKBRN4ZAGQP -> ../e8876a226237217ec61c4baf238a32992291d059fdac95ed6303bdff3f59cff5/diff

然后我们看看具体的目录下是什么,如果是最下面的是没有lower的

 1  ls /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/
 2 
 3 diff  link
 4 
 5 cat /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/link
 6 
 7 6Y5IM2XC7TSNIJZZFLJCS6I4I4
 8 
 9 
10 ls  /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/diff
11 
12 bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

lower从第二次开始

 1 //最底层
 2 cat 72dd847f8c4a40ce1762353c216d32e18db433b17c65e61ee1558758631fb59f/lower
 3 cat: 72dd847f8c4a40ce1762353c216d32e18db433b17c65e61ee1558758631fb59f/lower: 没有那个文件或目录
 4 
 5 //倒数第二次层
 6 cat 91af527ebbb6357fb1694334be10105edd07432da7cb901ef17ecdaf28944442/lower
 7 l/YSWCORVIDFAIEIAFPP5AWBJZ5G
 8 
 9 //倒数第三次
10 cat 76bc3e1bdecdd1da6ecfea3086d7fecefa589e567da864fd5a4b910c04568bbb/lower
11 l/N7S5NM6TVQ4X7NFK7ROIQ6JOAP:l/YSWCORVIDFAIEIAFPP5AWBJZ5G
12 
13 //倒数第四底层
14 cat 6d7bdb155539b21b411fe5a4b7ebd41a7bc92dfb5d0158b961622dee834e19d0/lower
15 l/Q7UBZ47OWOXEF4YL5POZBJ3UKY:l/N7S5NM6TVQ4X7NFK7ROIQ6JOAP:l/YSWCORVIDFAIEIAFPP5AWBJZ5G
16 
17 //镜像最上层
18 cat c249bc61bf63b4f39b316b30f0dbe83bc6b9425f6fc92b28dd9b36bf80308f5e/lower
19 l/JF7WPJE6K6CN5A7SSJEYDWWWMA:l/Q7UBZ47OWOXEF4YL5POZBJ3UKY:l/N7S5NM6TVQ4X7NFK7ROIQ6JOAP:l/YSWCORVIDFAIEIAFPP5AWBJZ5G

通过lower标识了镜像的父层的分层关联关系

diff记录了本层的信息，

1 ll  91af527ebbb6357fb1694334be10105edd07432da7cb901ef17ecdaf28944442/diff/
2 
3 drwxr-xr-x 4 root root 4096 1月  26 02:23 etc
4 drwxr-xr-x 2 root root 4096 1月  26 02:23 sbin
5 drwxr-xr-x 3 root root 4096 1月  24 06:49 usr
6 drwxr-xr-x 3 root root 4096 1月  24 06:49 var

如果是容器层，还会多一个merge层，这个和overlay的merge的概念是一样的。譬如我启动一个容器，并在var目录下创建aaaa文件可以看到最上层的读写层

 1 ll a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0/diff/var/
 2 
 3 -rw-r--r-- 1 root root 0 1月  31 12:23 aaaa
 4 
 5 
 6 ll a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0/merged/var/
 7 
 8 -rw-r--r-- 1 root root    0 1月  31 12:23 aaaa
 9 drwxr-xr-x 2 root root 4096 4月  13 2016 backups
10 drwxr-xr-x 5 root root 4096 1月  24 06:49 cache
11 drwxr-xr-x 1 root root 4096 2月   5 2016 lib
12 drwxrwsr-x 2 root ftp  4096 4月  13 2016 local
13 lrwxrwxrwx 1 root root    9 1月  24 06:49 lock -> /run/lock
14 drwxr-xr-x 4 root root 4096 1月  24 06:49 log
15 drwxrwsr-x 2 root mem  4096 1月  24 06:49 mail
16 drwxr-xr-x 2 root root 4096 1月  24 06:49 opt
17 lrwxrwxrwx 1 root root    4 1月  24 06:49 run -> /run
18 drwxr-xr-x 2 root root 4096 1月  24 06:49 spool
19 drwxrwxrwt 2 root root 4096 1月  24 06:49 tmp

这里如果启动容器还有一点需要介绍，你会看到多了一个”读写层-init”，这个只读层，它的目的是为了初始化容器配置信息，譬如hostname等信息

1 ll a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0-init/diff/etc/hostname 
2 -rwxr-xr-x 1 root root 0 1月  31 12:21 a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0-init/diff/etc/hostname

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查看mount信息

1  mount|grep overlay
2 overlay on /var/lib/docker/overlay2/a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0/merged type 
3 overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/EETCZ74DSQUEXSCTWSVYKD6RSA:
4 /var/lib/docker/overlay2/l/MOZ5Z5Y6HVYMH2C5H4HCP64VPX:/var/lib/docker/overlay2/l/JF7WPJE6K6CN5A7SSJEYDWWWMA:
5 /var/lib/docker/overlay2/l/Q7UBZ47OWOXEF4YL5POZBJ3UKY:/var/lib/docker/overlay2/l/N7S5NM6TVQ4X7NFK7ROIQ6JOAP:
6 /var/lib/docker/overlay2/l/YSWCORVIDFAIEIAFPP5AWBJZ5G,
7 upperdir=/var/lib/docker/overlay2/a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0/diff,
8 workdir=/var/lib/docker/overlay2/a1d281675ce2eacb0617b989ae846e29b8890954b8917b2919fbd025f537d7a0/work)

上面lowdir（只读层），第一个是最上层，譬如EETCZ74DSQUEXSCTWSVYKD6RSA，这个顺序很重要