Docker的overlay2學習

docker配置overlay2

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

[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目錄存放了短軟鏈接指向了CacheID目錄下的diff目錄，這是為了掛載命令不會超出長度

[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
...

最底層只有diff和link文件盏求，link文件內容是l目錄里面的縮寫鏈接名赊琳，實際就是對應diff目錄钳幅，diff目錄實際存放這一層的文件內容

[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

高位層有以上目錄，如果有多個低位層則會有l(wèi)ower文件，對應文件內容：

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

啟動容器后的目錄掛載情況：

[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層：
讀文件的時候催享，文件不在upperdir則從lowerdir讀，如果寫的文件不在uppderdir在lowerdir哟绊，則從lowerdir里面copy到upperdir因妙，不管文件多大，copy完再寫，刪除或者重命名鏡像層的文件都只是在容器層生成whiteout文件標志攀涵。

overlay2支持多個容器訪問相同文件時公用page cache铣耘，在copy_up的時候overlay2比aufs更快，因為aufs的層級多搜索會有延遲以故，而overlay2會有緩存機制蜗细。優(yōu)化方面可以用ssd，然后頻繁的io操作怒详，可以通過掛載volumes來做炉媒，繞過存儲驅動，而且可以多個容器共享數據棘利，持久化數據橱野。

overlay和overlay2的區(qū)別
overlay: 只掛載一層,其他層通過最高層通過硬連接形式共享(增加了磁盤inode的負擔)
overlay2: 逐層掛載

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