Service 的容器副本会 scale up/down ，会 failover，会在不同的主机上创建和销毁，这就引出一个问题，如果Service有要管理的数据，那么这些数据应该如何存放呢？
选项一：打包在容器里
显然不行，除非数据不会发生变化，否则，如何在多个副本间保持同步呢
选项二：数据放在Docker主机的本地目录中，通过 volume 映射到容器中
位于同一个主机的副本倒是可以共享这个volume，但是不同主机中的副本该如何同步呢？
选项三：利用 Docker 的 volume driver ，由外部的storage provider 管理和提供 volume，所有Docker 主机的volume将挂载到各个副本。
这是目前最好的方案了，volume 不依赖docker主机和容器，生命周期由storage provider 管理，volume 的高可用和数据有效性也全权由provider负责，Docker只管使用。
下面我们以 nfs 来实践第三种方案
host01 10.12.31.211 nfs-client
host02 10.12.31.212 nfs-client
host03 10.12.31.213 nfs-client + nfs-server（/var/nfs）
安装和配置 nfs-server
root@host03:~# apt install -y nfs-kernel-server
root@host03:~# mkdir /var/nfs
root@host03:~# cat /etc/exports
/var/nfs * (rw,sync,no_root_squash)
root@host03:~# systemctl restart nfs-kernel-server.service
安装和配置 nfs-client （需要在每台host都创建一遍 volume）
root@host01:~# apt install -y nfs-common
root@host01:~# docker volume create --driver local --opt type=nfs --opt o=addr=10.12.31.213,rw --opt device=:/var/nfs volume-nfs
root@host01:~# docker volume ls
DRIVER VOLUME NAME
local volume-nfs
root@host02:~# apt install -y nfs-common
root@host02:~# docker volume create --driver local --opt type=nfs --opt o=addr=10.12.31.213,rw --opt device=:/var/nfs volume-nfs
root@host01:~# docker volume ls
DRIVER VOLUME NAME
local volume-nfs
root@host03:~# apt install -y nfs-common
root@host03:~# docker volume create --driver local --opt type=nfs --opt o=addr=10.12.31.213,rw --opt device=:/var/nfs volume-nfs
root@host01:~# docker volume ls
DRIVER VOLUME NAME
local volume-nfs
创建Service 并挂载nfs 的volume，并验证
root@host03:~# docker service create --name my_web --publish 80:80 --mount type=volume,source=volume-nfs,destination=/usr/local/apache2/htdocs --replicas 2 httpd
z3ojyj6n5ibpyh4ab49664pvn
overall progress: 2 out of 2 tasks
1/2: running
2/2: running
verify: Service converged
root@host03:~# cat /var/nfs/index.html
docker swarm nfs volume test
root@host03:~# curl http://10.12.31.211
docker swarm nfs volume test
root@host03:~# curl http://10.12.31.212
docker swarm nfs volume test
root@host03:~# curl http://10.12.31.213
docker swarm nfs volume test
root@host03:~# docker service ps my_web
ID NAME IMAGE NODE DESIRED STATE CURRENT STATE ERROR PORTS
a2v73m955o9j my_web.1 httpd:latest host01 Running Running 4 minutes ago
uzdfouv3s2bz my_web.2 httpd:latest host02 Running Running 4 minutes ago
root@host01:~# docker exec -it my_web.1.a2v73m955o9js3fbihf0dwei6 cat /usr/local/apache2/htdocs/index.html
docker swarm nfs volume test
root@host01:~# docker volume inspect volume-nfs
[
{
"CreatedAt": "2019-05-15T21:11:42+08:00",
"Driver": "local",
"Labels": {},
"Mountpoint": "/var/lib/docker/volumes/volume-nfs/_data",
"Name": "volume-nfs",
"Options": {
"device": ":/var/nfs",
"o": "addr=10.12.31.213,rw",
"type": "nfs"
},
"Scope": "local"
}
]
root@host01:~# docker inspect my_web.1.a2v73m955o9js3fbihf0dwei6 | jq .[0].Mounts
[
{
"Type": "volume",
"Name": "volume-nfs",
"Source": "/var/lib/docker/volumes/volume-nfs/_data",
"Destination": "/usr/local/apache2/htdocs",
"Driver": "local",
"Mode": "z",
"RW": true,
"Propagation": ""
}
]
root@host02:~# docker exec -it my_web.2.uzdfouv3s2bz0ohyahjeeyqao cat /usr/local/apache2/htdocs/index.html
docker swarm nfs volume test
root@host02:~# docker volume inspect volume-nfs
[
{
"CreatedAt": "2019-05-15T21:11:42+08:00",
"Driver": "local",
"Labels": {},
"Mountpoint": "/var/lib/docker/volumes/volume-nfs/_data",
"Name": "volume-nfs",
"Options": {
"device": ":/var/nfs",
"o": "addr=10.12.31.213,rw",
"type": "nfs"
},
"Scope": "local"
}
]
root@host02:~# docker inspect my_web.2.uzdfouv3s2bz0ohyahjeeyqao | jq .[0].Mounts
[
{
"Type": "volume",
"Name": "volume-nfs",
"Source": "/var/lib/docker/volumes/volume-nfs/_data",
"Destination": "/usr/local/apache2/htdocs",
"Driver": "local",
"Mode": "z",
"RW": true,
"Propagation": ""
}
]