容器及其layer存储分析
本文接上篇镜像及其layer存储分析分析容器描述信息及其读写层的存放位置。
容器目录
默认情况下，容器相关的数据保存在目录/var/lib/docker/containers下面。
以nginx:latest镜像创建一个容器，但并不启动它。可以发现一个和容器ID同名的目录出现在了/var/lib/docker/containers下面。
[root@localhost containers]# pwd
/var/lib/docker/containers
[root@localhost containers]# docker create nginx
4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c
[root@localhost containers]# ls
4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c
我们知道创建一个容器，相当于在镜像顶层之上增加了一个新的layer（可读可写），那这个新的layer在哪里呢？
这个layer和镜像的layer一样都保存在/var/lib/docker/image/overlay2/layerdb下面。不同的是，镜像的layer保存在子目录sha256下面，而容器layer保存在子目录mounts下面。
[root@localhost mounts]# pwd
/var/lib/docker/image/overlay2/layerdb/mounts
[root@localhost mounts]# ll
total 0
drwxr-xr-x 2 root root 51 Nov 9 09:21 4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c
[root@localhost mounts]# cd 4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c/
[root@localhost 4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c]# ls
init-id mount-id parent
容器layer包含3个文件：
- parent：引用的镜像的最顶层的CHAIN-ID。
- init-id：在storage-driver中init层的目录名称
- mount-id：在storage-driver中容器层的目录名称
镜像的各个layer以cache-id关联到storage-driver系统中，容器的layer一个init-id和mount-id关联到storage-driver系统中。
[root@localhost 4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c]# cat parent
sha256:160a8bd939a9421818f499ba4fbfaca3dd5c86ad7a6b97b6889149fd39bd91dd
[root@localhost 4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c]# cat init-id
89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init
[root@localhost 4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c]# cat mount-id
89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06
查看storage-driver(overlay2)下面出现了2个文件夹，文件夹名称和init-id和mount-id内容一致。
[root@localhost overlay2]# pwd
/var/lib/docker/overlay2
[root@localhost overlay2]# ll | grep 89dbd3
drwx------ 5 root root 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06
drwx------ 5 root root 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init
再查看文件夹下各主要文件的内容：
[root@localhost overlay2]# ls 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init
diff link lower merged work
[root@localhost overlay2]# cat 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init/link
FEWHQ7576N2NZ4QEAGUFHVHSY5
[root@localhost overlay2]# cat 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init/lower
l/V47ZAVU47VNVE3B7TVCZIGKGOV:l/NUMROUBW7TBBLT56IWT4OL53EZ:l/MVB5STDEZWHO54D642CLYZYNEH
[root@localhost overlay2]# ls 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06
diff link lower merged work
[root@localhost overlay2]# cat 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06/link
YMY2R223JESTB3XLXXGGSIXARB
[root@localhost overlay2]# cat 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06/lower
l/FEWHQ7576N2NZ4QEAGUFHVHSY5:l/V47ZAVU47VNVE3B7TVCZIGKGOV:l/NUMROUBW7TBBLT56IWT4OL53EZ:l/MVB5STDEZWHO54D642CLYZYNEH
通过检查lower文件可以发现：89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init层的lowerdir是其依赖镜像的3个层；而89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06的lowerdir包括4个层，分别是89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06-init层以及镜像的3个层。
启动容器
容器启动之前可以发现当前layer是空的，并且文件系统没有挂载，merged文件夹下面也是空的。
[root@localhost 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06]# ll diff/
total 0
[root@localhost 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06]# ll merged/
total 0
然后启动容器：
[root@localhost 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06]# ll diff/
total 0
drwxr-xr-x. 3 root root 38 Nov 9 10:03 run
drwxr-xr-x. 3 root root 19 Apr 26 2018 var
[root@localhost 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06]# ll merged/
total 8
drwxr-xr-x. 2 root root 4096 Apr 26 2018 bin
drwxr-xr-x. 2 root root 6 Feb 24 2018 boot
drwxr-xr-x. 1 root root 43 Nov 9 09:21 dev
drwxr-xr-x. 1 root root 66 Nov 9 09:21 etc
drwxr-xr-x. 2 root root 6 Feb 24 2018 home
drwxr-xr-x. 1 root root 45 Apr 26 2018 lib
drwxr-xr-x. 2 root root 34 Apr 26 2018 lib64
drwxr-xr-x. 2 root root 6 Apr 26 2018 media
drwxr-xr-x. 2 root root 6 Apr 26 2018 mnt
drwxr-xr-x. 2 root root 6 Apr 26 2018 opt
drwxr-xr-x. 2 root root 6 Feb 24 2018 proc
drwx------. 2 root root 37 Apr 26 2018 root
drwxr-xr-x. 1 root root 38 Nov 9 10:03 run
drwxr-xr-x. 2 root root 4096 Apr 26 2018 sbin
drwxr-xr-x. 2 root root 6 Apr 26 2018 srv
drwxr-xr-x. 2 root root 6 Feb 24 2018 sys
drwxrwxrwt. 1 root root 6 Apr 30 2018 tmp
drwxr-xr-x. 1 root root 66 Apr 26 2018 usr
drwxr-xr-x. 1 root root 19 Apr 26 2018 var
此时发现，文件系统已经生效，并且overlay2文件系统已经挂载。
[root@dqvm 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06]# df -h
Filesystem Size Used Avail Use% Mounted on
//忽略无关文件系统
overlay 26G 18G 8.5G 68% /var/lib/docker/overlay2/89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06/merged
shm 64M 0 64M 0% /var/lib/docker/containers/4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c/shm
挂载的详情信息：
[root@dqvm 89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06]# mount
overlay on /var/lib/docker/overlay2/89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06/merged type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/FEWHQ7576N2NZ4QEAGUFHVHSY5:/var/lib/docker/overlay2/l/V47ZAVU47VNVE3B7TVCZIGKGOV:/var/lib/docker/overlay2/l/NUMROUBW7TBBLT56IWT4OL53EZ:/var/lib/docker/overlay2/l/MVB5STDEZWHO54D642CLYZYNEH,upperdir=/var/lib/docker/overlay2/89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06/diff,workdir=/var/lib/docker/overlay2/89dbd3a7893baa0efbf8f77ac58b3192561fd62891531eb73d68feb1e600af06/work)
shm on /var/lib/docker/containers/4f0e4cc32ad2e846f0df0ca08df4be38b7a17876f66dfe055feab55ac527be3c/shm type tmpfs (rw,nosuid,nodev,noexec,relatime,size=65536k)