docker中数据卷探究与总结

Posted 2021-02-09

为实现持久化，docker引入了挂载数据卷的方法，以实现数据的可持久化。具体是以下三种：
一.通过docker run命令
命令：
docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test.txt:/var/spool/cron/crontabs/root openfrontier/gerrit:2.15.3
此时将宿主机的/data/mysql1挂载到容器的/var/lib/mysql下，这里需要注意的还分以下几种情况：
A.挂载本地不存在的文件，到容器中已存在的文件，无法启动
docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/review_site_test/root:/var/spool/cron/crontabs/root openfrontier/gerrit:2.15.3

结果无法启动

B.挂载本地不存在的文件夹，到容器内已存在的文件夹，可以启动，但会清空容器内的指定目录/var/spool/cron/crontabs下的所有文件
docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/review_site_test:/var/spool/cron/crontabs openfrontier/gerrit:2.15.3

结果可以启动

但需要注意的是挂载本地不存在的文件夹过去，会清空当前容器的指定文件夹。如果容器中对该文件夹有启动依赖，例如上面的gerrit镜像:
使用下面的命令就无法启动：
docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/review-site21a:/usr/share/gitweb openfrontier/gerrit:2.15.3

原因:gerrit依赖于gitweb

C.挂载本地存在的文件，到容器内指定文件，可以启动，该文件会覆盖你指定的文件
[[email protected] docker]# cat test.txt
askdkjajksdk

[[email protected] docker]# docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test.txt:/var/spool/cron/crontabs/root openfrontier/gerrit:2.15.3
c86aac5a73d551163c262bdf4db8cc4253322ac4c41d2d2418675c5b4f63e62a

bash-4.4# cd /var/spool/cron/crontabs
bash-4.4# ls
root
bash-4.4# vim root
bash: vim: command not found
bash-4.4# cat root
askdkjajksdk

D.挂载本地存在的文件夹test，到容器里已存在的文件夹，发现test目录中的文件，直接到容器内指定目录下
[[email protected] docker]# docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test:/var/spool/cron/crontabs openfrontier/gerrit:2.15.3
9c32db2cff1d8d7ed3636a812302470d9408f4c60786067a57da770b930ae01f
[[email protected] docker]# docker exec -it gerrit /bin/bash
bash-4.4# cd /var/spool/cron/
bash-4.4# ls
crontabs
bash-4.4# cd crontabs
bash-4.4# ls
test.txt
bash-4.4# vim test.txt
bash: vim: command not found
bash-4.4# cat test.txt
askdkjajksdk

F.挂载本地存在的文件夹test，到容器里不存在的文件夹，会在容器内创建该test文件夹
E的变种
[[email protected] docker]# docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test:/var/spool/cron/test openfrontier/gerrit:2.15.3
53fb24583eb65278599975ca515000080267e8348e945f1fcbd4fc4f8e6cb165
[[email protected] docker]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
53fb24583eb6 openfrontier/gerrit:2.15.3 "/gerrit-entrypoin..." 4 seconds ago Up 2 seconds 0.0.0.0:9001->8080/tcp, 0.0.0.0:29419->29418/tcp gerrit
d114a167bbcc nginx:1.15.2-alpine "nginx -g ‘daemon ..." 41 hours ago Exited (0) 35 minutes ago test_nginx_1
641508207b44 mysql:5.7 "docker-entrypoint..." 41 hours ago Exited (0) 35 minutes ago test_db_1
[[email protected] docker]# docker exec -it gerrit /bin/bash
bash-4.4# cd /var/spool/cron/
bash-4.4# ls
crontabs test
bash-4.4# cd test/
bash-4.4# ls
test.txt

G.挂载本地是文件test.txt，到容器内指定的目录crontabs，报错
[[email protected] docker]# docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test/test.txt:/var/spool/cron/crontabs openfrontier/gerrit:2.15.3
e63487c08abce10c8bdbcf834714ad0994a93118abdd5126969856f862ca7f0b
/usr/bin/docker-current: Error response from daemon: oci runtime error: container_linux.go:247: starting container process caused "process_linux.go:364: container init caused "rootfs_linux.go:54: mounting \"/home/docker/test/test.txt\" to rootfs \"/var/lib/docker/overlay2/5b0428b09a0727e114f06d312106e4fb28bc31d728abd27dab2f30328497c3c7/merged\" at \"/var/lib/docker/overlay2/5b0428b09a0727e114f06d312106e4fb28bc31d728abd27dab2f30328497c3c7/merged/etc/crontabs\" caused \"not a directory\"""
: Are you trying to mount a directory onto a file (or vice-versa)? Check if the specified host path exists and is the expected type.

看过第一种肯定有一些疑惑，因为有些镜像是违背第一种原则，即宿主机不存在某个目录，-v参数指定的宿主机目录:容器目录，容器目录里的东西立马都到宿主机指定的目录下了，不是因为宿主机的空目录覆盖掉容器的指定目录吗？怎么容器目录里的东西全都过来呢？
例如你用这条命令试试：
docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test1:/var/gerrit/review_site openfrontier/gerrit:2.15.3

别着急等会儿解释。

二.通过dockerfile中的volume创建挂载点
下面我们看下mysql的dockerfile

FROM debian:stretch-slim

# add our user and group first to make sure their IDs get assigned consistently, regardless of whatever dependencies get added
RUN groupadd -r mysql && useradd -r -g mysql mysql

RUN apt-get update && apt-get install -y --no-install-recommends gnupg dirmngr && rm -rf /var/lib/apt/lists/*

# add gosu for easy step-down from root
ENV GOSU_VERSION 1.7
RUN set -x     && apt-get update && apt-get install -y --no-install-recommends ca-certificates wget && rm -rf /var/lib/apt/lists/*     && wget -O /usr/local/bin/gosu "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture)"     && wget -O /usr/local/bin/gosu.asc "https://github.com/tianon/gosu/releases/download/$GOSU_VERSION/gosu-$(dpkg --print-architecture).asc"     && export GNUPGHOME="$(mktemp -d)"     && gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys B42F6819007F00F88E364FD4036A9C25BF357DD4     && gpg --batch --verify /usr/local/bin/gosu.asc /usr/local/bin/gosu     && gpgconf --kill all     && rm -rf "$GNUPGHOME" /usr/local/bin/gosu.asc     && chmod +x /usr/local/bin/gosu     && gosu nobody true     && apt-get purge -y --auto-remove ca-certificates wget

RUN mkdir /docker-entrypoint-initdb.d

RUN apt-get update && apt-get install -y --no-install-recommends # for MYSQL_RANDOM_ROOT_PASSWORD
        pwgen # for mysql_ssl_rsa_setup
        openssl # FATAL ERROR: please install the following Perl modules before executing /usr/local/mysql/scripts/mysql_install_db:
# File::Basename
# File::Copy
# Sys::Hostname
# Data::Dumper
        perl     && rm -rf /var/lib/apt/lists/*

RUN set -ex; # gpg: key 5072E1F5: public key "MySQL Release Engineering <[email protected]>" imported
    key=‘A4A9406876FCBD3C456770C88C718D3B5072E1F5‘;     export GNUPGHOME="$(mktemp -d)";     gpg --batch --keyserver ha.pool.sks-keyservers.net --recv-keys "$key";     gpg --batch --export "$key" > /etc/apt/trusted.gpg.d/mysql.gpg;     gpgconf --kill all;     rm -rf "$GNUPGHOME";     apt-key list > /dev/null

ENV MYSQL_MAJOR 5.7
ENV MYSQL_VERSION 5.7.25-1debian9

RUN echo "deb http://repo.mysql.com/apt/debian/ stretch mysql-${MYSQL_MAJOR}" > /etc/apt/sources.list.d/mysql.list

# the "/var/lib/mysql" stuff here is because the mysql-server postinst doesn‘t have an explicit way to disable the mysql_install_db codepath besides having a database already "configured" (ie, stuff in /var/lib/mysql/mysql)
# also, we set debconf keys to make APT a little quieter
RUN {         echo mysql-community-server mysql-community-server/data-dir select ‘‘;         echo mysql-community-server mysql-community-server/root-pass password ‘‘;         echo mysql-community-server mysql-community-server/re-root-pass password ‘‘;         echo mysql-community-server mysql-community-server/remove-test-db select false;     } | debconf-set-selections     && apt-get update && apt-get install -y mysql-server="${MYSQL_VERSION}" && rm -rf /var/lib/apt/lists/*     && rm -rf /var/lib/mysql && mkdir -p /var/lib/mysql /var/run/mysqld     && chown -R mysql:mysql /var/lib/mysql /var/run/mysqld # ensure that /var/run/mysqld (used for socket and lock files) is writable regardless of the UID our mysqld instance ends up having at runtime
    && chmod 777 /var/run/mysqld # comment out a few problematic configuration values
    && find /etc/mysql/ -name ‘*.cnf‘ -print0         | xargs -0 grep -lZE ‘^(bind-address|log)‘         | xargs -rt -0 sed -Ei ‘s/^(bind-address|log)/#&/‘ # don‘t reverse lookup hostnames, they are usually another container
    && echo ‘[mysqld]
skip-host-cache
skip-name-resolve‘ > /etc/mysql/conf.d/docker.cnf

VOLUME /var/lib/mysql

COPY docker-entrypoint.sh /usr/local/bin/
RUN ln -s usr/local/bin/docker-entrypoint.sh /entrypoint.sh # backwards compat
ENTRYPOINT ["docker-entrypoint.sh"]

EXPOSE 3306 33060
CMD ["mysqld"]

很明显有个标签:
VOLUME
这个标签我理解是将容器目录暴露出去，简单理解就是容器中的目录可以挂载到宿主机的某个匿名目录，一般是在docker的主程序目录下，一般没做修改的话就是/var/lib/docker下，我这里是/data/docker
来看下具体命令：

那么我们到标红的那段里面看看，是否和/var/lib/mysql一样呢？结果是一样的

那老是匿名不是个事，我们能指定吗？
第一种方法-v不就行了吗？
下面试试：

结果发现是可以了，/var/lib/mysql里的东西全部到/data/mysql1里面了，这里其实是容器的目录复制到指定目录，宿主机的指定目录再挂载到容器的指定目录中，这理解起来可能有些困难，但是确实是这样。
那么我们就很好解决了上面gerrit数据卷的问题了，因为它的dockerfile里面也是有VOLUME标签的。

三.使用--volumes-from参数指定挂载的容器的宿主机目录
上图，不多bb

可以发现mysql1和mysql用的是用一个宿主机目录/data/mysql1

好了，数据卷东西就写到这了