環境
- virtual box 6.1
- centos 7.8
- docker 19.03
overlay 掛載
先在宿主機上模擬 overlay 掛載。
建立需要的目錄
[root@master overlay-fs]# tree .
.
├── lower1
│ └── l1.txt
├── lower2
│ ├── l2.txt
│ └── l3.txt
├── lower3
│ └── l3.txt
├── merged
├── upper
└── work
6 directories, 4 files
[root@master overlay-fs]# cat lower1/l1.txt
lower1
[root@master overlay-fs]# cat lower2/l2.txt
lower2
[root@master overlay-fs]# cat lower2/l3.txt
lower3 in lower 2
[root@master overlay-fs]# cat lower3/l3.txt
lower3
[root@master overlay-fs]#
執行掛載命令
[root@master overlay-fs]# mount -t overlay overlay -o lowerdir=lower1:lower2:lower3,upperdir=upper,workdir=work merged/
[root@master overlay-fs]# ls merged/
l1.txt l2.txt l3.txt
[root@master overlay-fs]# cat merged/l2.txt
lower2
[root@master overlay-fs]# cat merged/l3.txt
lower3 in lower 2
[root@master overlay-fs]#
查看掛載信息
[root@master overlay-fs]# mount | grep overlay-fs
overlay on /root/overlay-fs/merged type overlay (rw,relatime,seclabel,lowerdir=lower1:lower2:lower3,upperdir=upper,workdir=work)
鏡像層
獲取鏡像
[root@master ~]# docker pull ubuntu
Using default tag: latest
latest: Pulling from library/ubuntu
6a5697faee43: Pull complete
ba13d3bc422b: Pull complete
a254829d9e55: Pull complete
Digest: sha256:fff16eea1a8ae92867721d90c59a75652ea66d29c05294e6e2f898704bdb8cf1
Status: Downloaded newer image for ubuntu:latest
docker.io/library/ubuntu:latest
[root@master ~]#
可以看到，總共從服務器拉取了三層。
查看構建紀錄
[root@master ~]# docker history ubuntu:latest
IMAGE CREATED CREATED BY SIZE COMMENT
d70eaf7277ea 4 weeks ago /bin/sh -c #(nop) CMD ["/bin/bash"] 0B
<missing> 4 weeks ago /bin/sh -c mkdir -p /run/systemd && echo 'do… 7B
<missing> 4 weeks ago /bin/sh -c [ -z "$(apt-get indextargets)" ] 0B
<missing> 4 weeks ago /bin/sh -c set -xe && echo '#!/bin/sh' > /… 811B
<missing> 4 weeks ago /bin/sh -c #(nop) ADD file:435d9776fdd3a1834… 72.9MB
沒有修改的話，不會生成鏡像層。
查看鏡像層
[root@master ~]# docker image inspect ubuntu:latest -f '{{json .GraphDriver}}' | jq
{
"Data": {
"LowerDir": "/var/lib/docker/overlay2/d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191/diff:/var/lib/docker/overlay2/cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8/diff",
"MergedDir": "/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/merged",
"UpperDir": "/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/diff",
"WorkDir": "/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/work"
},
"Name": "overlay2"
}
[root@master ~]# docker image inspect ubuntu:latest -f '{{json .RootFS}}' | jq
{
"Type": "layers",
"Layers": [
"sha256:47dde53750b4a8ed24acebe52cf31ad131e73a9611048fc2f92c9b6274ab4bf3",
"sha256:0c2689e3f9206b1c4adfb16a1976d25bd270755e734588409b31ef29e3e756d6",
"sha256:cc9d18e90faad04bc3893cfaa50b7846ee75f48f5b8377a213fa52af2189095c"
]
}
從 RootFS 和 GraphDriver 可以看到，鏡像包括三層，其中 lowerdir 有兩層。
鏡像層詳細
[root@master ~]# ls /var/lib/docker/overlay2/cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8/diff
bin boot dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
[root@master ~]# ls /var/lib/docker/overlay2/d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191/diff
etc usr var
[root@master ~]# ls /var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/diff
run
[root@master ~]#
[root@master ~]# ls /var/lib/docker/overlay2/
904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8 l
backingFsBlockDev d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191
容器層
啟動容器
[root@master ~]# docker run -d ubuntu:latest sleep 3600
ea3e818e7690523e5758bf529bc50bd92d68ad26e02f4953bdb68f9bc5ecd567
[root@master ~]#
查看鏡像層
[root@master ~]# docker container inspect ea -f '{{json .GraphDriver}}' | jq
{
"Data": {
"LowerDir": "/var/lib/docker/overlay2/1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402-init/diff:/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/diff:/var/lib/docker/overlay2/d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191/diff:/var/lib/docker/overlay2/cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8/diff",
"MergedDir": "/var/lib/docker/overlay2/1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402/merged",
"UpperDir": "/var/lib/docker/overlay2/1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402/diff",
"WorkDir": "/var/lib/docker/overlay2/1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402/work"
},
"Name": "overlay2"
}
可以看到，lower的底下三層剛好是鏡像的三層，除此之外，還有一個 init 層，在 upperdir 中有一個可寫的容器層。
整個容器的層數可以分為三個部分。第一個部分是只讀的鏡像層，對應是 ubuntu 鏡像的三層。第二部分是 init 層。第三層就是容器的可讀寫層。
init 層
[root@master ~]# ls /var/lib/docker/overlay2/
1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402 backingFsBlockDev l
1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402-init cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8
904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191
[root@master ~]#
[root@master ~]# ls /var/lib/docker/overlay2/1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402-init/diff/etc/
hostname hosts mtab resolv.conf
[root@master ~]#
init 層是一個以“-init”結尾的層，夾在只讀層和讀寫層之間。Init 層是 Docker 項目單獨生成的一個內部層，專門用來存放 /etc/hosts、/etc/resolv.conf 等信息。需要這樣一層的原因是，這些文件本來屬於只讀的 Ubuntu 鏡像的一部分，但是用戶往往需要在啟動容器時寫入一些指定的值比如 hostname，所以就需要在可讀寫層對它們進行修改。可是，這些修改往往只對當前的容器有效，我們並不希望執行 docker commit 時，把這些信息連同可讀寫層一起提交掉。所以，Docker 做法是，在修改了這些文件之后，以一個單獨的層掛載了出來。而用戶執行 docker commit 只會提交可讀寫層，所以是不包含這些內容的。
容器可讀寫層
[root@master 1e8611585b1c2a4d6066d0829c70a2fb7b0e6da64e9015e81201c0b5c7fa9402]# cd diff/
[root@master diff]# ls
[root@master diff]# docker exec -it ea bash
root@ea3e818e7690:/# ls
bin boot dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@ea3e818e7690:/# touch jiangbo.txt
root@ea3e818e7690:/# exit
exit
[root@master diff]# ls
jiangbo.txt root
[root@master diff]#
RootFS 和 GraphDriver 的關聯
[root@master sha256]# cd /var/lib/docker/image/overlay2/layerdb/sha256
[root@master sha256]# ll
total 0
drwx------. 2 root root 71 Nov 25 00:03 47dde53750b4a8ed24acebe52cf31ad131e73a9611048fc2f92c9b6274ab4bf3
drwx------. 2 root root 85 Nov 25 00:03 7011438f48b79cbf5fce3bfba74aed2e53fe5fe6a3b7fd6fe03018d28caee7a3
drwx------. 2 root root 85 Nov 25 00:03 778d52487737cf5362fd95086fa55793001dbc7b331344a540a594824e2994fd
[root@master sha256]# cat 47dde53750b4a8ed24acebe52cf31ad131e73a9611048fc2f92c9b6274ab4bf3/cache-id
cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8
[root@master sha256]# cat 47dde53750b4a8ed24acebe52cf31ad131e73a9611048fc2f92c9b6274ab4bf3/diff
sha256:47dde53750b4a8ed24acebe52cf31ad131e73a9611048fc2f92c9b6274ab4bf3
[root@master sha256]# cat 7011438f48b79cbf5fce3bfba74aed2e53fe5fe6a3b7fd6fe03018d28caee7a3/cache-id
d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191
[root@master sha256]# cat 7011438f48b79cbf5fce3bfba74aed2e53fe5fe6a3b7fd6fe03018d28caee7a3/diff
sha256:0c2689e3f9206b1c4adfb16a1976d25bd270755e734588409b31ef29e3e756d6
[root@master sha256]# cat 778d52487737cf5362fd95086fa55793001dbc7b331344a540a594824e2994fd/cache-id
904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b
[root@master sha256]# cat 778d52487737cf5362fd95086fa55793001dbc7b331344a540a594824e2994fd/diff
sha256:cc9d18e90faad04bc3893cfaa50b7846ee75f48f5b8377a213fa52af2189095c
[root@master sha256]#
{
"Data": {
"LowerDir": "/var/lib/docker/overlay2/d4ae655796da84cd446b1d9b3f4af81a2128e0dd4f6bb7f4549e877847ac2191/diff:/var/lib/docker/overlay2/cc17e7fa7371b562c3638dccc2c19439d4a7900e8b9e1207bcaa767a2bfde6f8/diff",
"MergedDir": "/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/merged",
"UpperDir": "/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/diff",
"WorkDir": "/var/lib/docker/overlay2/904fc870070ff53322419097027f739bc558c2e89443ee7530cd850143e6041b/work"
},
"Name": "overlay2"
}
[root@master ~]# docker image inspect ubuntu:latest -f '{{json .RootFS}}' | jq
{
"Type": "layers",
"Layers": [
"sha256:47dde53750b4a8ed24acebe52cf31ad131e73a9611048fc2f92c9b6274ab4bf3",
"sha256:0c2689e3f9206b1c4adfb16a1976d25bd270755e734588409b31ef29e3e756d6",
"sha256:cc9d18e90faad04bc3893cfaa50b7846ee75f48f5b8377a213fa52af2189095c"
]
}
總結
介紹了 overlay 的實現方式，以及在容器中的如何處理文件的增刪改查等。