docker中數據卷探究與總結
一.通過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
此時將宿主機的/data/mysql1掛載到容器的/var/lib/mysql下，這裏需要註意的還分以下幾種情況：
A.掛載本地不存在的文件，到容器中已存在的文件，無法啟動
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
結果無法啟動
B.掛載本地不存在的文件夾，到容器內已存在的文件夾，可以啟動，但會清空容器內的指定目錄/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
結果可以啟動
但需要註意的是掛載本地不存在的文件夾過去，會清空當前容器的指定文件夾。如果容器中對該文件夾有啟動依賴，例如上面的gerrit鏡像:
使用下面的命令就無法啟動：
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依賴於gitweb
C.掛載本地存在的文件，到容器內指定文件，可以啟動，該文件會覆蓋你指定的文件
[root@zzw docker]# cat test.txt
askdkjajksdk
[root@zzw 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.掛載本地存在的文件夾test，到容器裏已存在的文件夾，發現test目錄中的文件，直接到容器內指定目錄下
[root@zzw 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
[root@zzw 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.掛載本地存在的文件夾test，到容器裏不存在的文件夾，會在容器內創建該test文件夾
E的變種
[root@zzw 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
[root@zzw 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
[root@zzw 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.掛載本地是文件test.txt，到容器內指定的目錄crontabs，報錯
[root@zzw 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.
看過第一種肯定有一些疑惑，因為有些鏡像是違背第一種原則，即宿主機不存在某個目錄，-v參數指定的宿主機目錄:容器目錄，容器目錄裏的東西立馬都到宿主機指定的目錄下了，不是因為宿主機的空目錄覆蓋掉容器的指定目錄嗎？怎麽容器目錄裏的東西全都過來呢？
例如你用這條命令試試：
docker run -dit --name gerrit -p 9001:8080 -p 29419:29418 -v /home/docker/test1:/var/gerrit/review_site openfrontier/gerrit:2.15.3
別著急等會兒解釋。
二.通過dockerfile中的volume創建掛載點
下面我們看下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]\nskip-host-cache\nskip-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"]很明顯有個標簽:
VOLUME
這個標簽我理解是將容器目錄暴露出去，簡單理解就是容器中的目錄可以掛載到宿主機的某個匿名目錄，一般是在docker的主程序目錄下，一般沒做修改的話就是/var/lib/docker下，我這裏是/data/docker
來看下具體命令：
那麽我們到標紅的那段裏面看看，是否和/var/lib/mysql一樣呢？結果是一樣的
那老是匿名不是個事，我們能指定嗎？
第一種方法-v不就行了嗎？
下面試試：
結果發現是可以了，/var/lib/mysql裏的東西全部到/data/mysql1裏面了，這裏其實是容器的目錄復制到指定目錄，宿主機的指定目錄再掛載到容器的指定目錄中，這理解起來可能有些困難，但是確實是這樣。
那麽我們就很好解決了上面gerrit數據卷的問題了，因為它的dockerfile裏面也是有VOLUME標簽的。
三.使用--volumes-from參數指定掛載的容器的宿主機目錄
上圖，不多bb
可以發現mysql1和mysql用的是用一個宿主機目錄/data/mysql1
好了，數據卷東西就寫到這了
docker中數據卷探究與總結