Docker數據卷掛載命令volume(-v)與mount的總結

前言

用戶可以通過docker run的--volume/-v或--mount選項來創建帶有數據卷的容器，但這兩個選項有些微妙的差異，在這裏總結梳理一下。

命令用法

--volume(-v)

參數--volume（或簡寫爲-v）只能創建bind mount。示例：

docker run --name $CONTAINER_NAME -it \
-v $PWD/$CONTAINER_NAME/app:/app:rw \
-v $PWD/$CONTAINER_NAME/data:/data:ro \
avocado-cloud:latest /bin/bash

註釋：

• 命令格式：[[HOST-DIR:]CONTAINER-DIR[:OPTIONS]]]
• 如果指定HOST-DIR則必須是絕對路徑，如果路徑不存在則會自動創建
• 實例中的rw爲讀寫，ro爲只讀

--mount

參數--mount默認情況下用來掛載volume，但也可以用來創建bind mount和tmpfs。如果不指定type選項，則默認爲掛載volume，volume是一種更爲靈活的數據管理方式，volume可以通過docker volume命令集被管理。示例：

docker run --name $CONTAINER_NAME -it \
--mount type=bind,source=$PWD/$CONTAINER_NAME/app,destination=/app \
--mount source=${CONTAINER_NAME}-data,destination=/data,readonly \
avocado-cloud:latest /bin/bash

註釋：

• 掛載volume命令格式：[type=volume,]source=my-volume,destination=/path/in/container[,...]
• 創建bind mount命令格式：type=bind,source=/path/on/host,destination=/path/in/container[,...]
• 如果創建bind mount並指定source則必須是絕對路徑，且路徑必須已經存在
• 示例中readonly表示只讀

差異總結

創建bind mount和掛載volume的比較

對比項	bind mount	volume
Source位置	用戶指定	/var/lib/docker/volumes/
Source爲空	覆蓋dest爲空	保留dest內容
Source非空	覆蓋dest內容	覆蓋dest內容
Source種類	文件或目錄	只能是目錄
可移植性	一般（自行維護）	強（docker託管）
宿主直接訪問	容易（僅需chown）	受限（需登陸root用戶）*

*註釋：Docker無法簡單地通過sudo chown someuser: -R /var/lib/docker/volumes/somevolume來將volume的內容開放給主機上的普通用戶訪問，如果開放更多權限則有安全風險。而這點上Podman的設計就要理想得多，volume存放在$HOME/.local/share/containers/storage/volumes/路徑下，即提供了便捷性，又保障了安全性。無需root權限即可運行容器，這正是Podman的優勢之一，實際使用過程中的確受益良多。

創建bind mount時使用--volume和--mount的比較

對比項	--volume 或 -v	--mount type=bind
如果主機路徑不存在	自動創建	命令報錯

官方文檔

DOCKER(1)                          JUNE 2014                         DOCKER(1)

NAME
       docker-run - Run a command in a new container

SYNOPSIS
       docker run
       [--mount[=[MOUNT]]]
       [-v|--volume[=[[HOST-DIR:]CONTAINER-DIR[:OPTIONS]]]]
       IMAGE

OPTIONS
       --mount type=TYPE,TYPE-SPECIFIC-OPTION[,...]
          Attach a filesystem mount to the container

       Current supported mount TYPES are bind, volume, and tmpfs.

       e.g.

       type=bind,source=/path/on/host,destination=/path/in/container

       type=volume,source=my-volume,destination=/path/in/container,volume-label="color=red",volume-label="shape=round"

       type=tmpfs,tmpfs-size=512M,destination=/path/in/container

       Common Options:

              · src, source: mount source spec for bind and volume. Mandatory
                for bind.

              · dst, destination, target: mount destination spec.

              · ro, readonly: true or false (default).

       Note: setting readonly for a bind mount does not make its submounts
          read-only on the current Linux implementation. See also
       bind-nonrecursive.

       Options specific to bind:

              · bind-propagation: shared, slave, private, rshared, rslave, or
                rprivate(default). See also mount(2).

              · consistency: consistent(default), cached, or delegated.
                Currently, only effective for Docker for Mac.

              · bind-nonrecursive: true or false (default). If set to true,
                submounts are not recursively bind-mounted. This option is
                useful for readonly bind mount.

       Options specific to volume:

              · volume-driver: Name of the volume-driver plugin.

              · volume-label: Custom metadata.

              · volume-nocopy: true(default) or false. If set to false, the
                Engine copies existing files and directories under the
                mount-path into the volume, allowing the host to access them.

              · volume-opt: specific to a given volume driver.
              
       Options specific to tmpfs:

              · tmpfs-size: Size of the tmpfs mount in bytes. Unlimited by
                default in Linux.

              · tmpfs-mode: File mode of the tmpfs in octal. (e.g. 700 or
                0700.) Defaults to 1777 in Linux.

       -v|--volume[=[[HOST-DIR:]CONTAINER-DIR[:OPTIONS]]]
          Create a bind mount. If you specify, -v /HOST-DIR:/CONTAINER-DIR,
       Docker
          bind mounts /HOST-DIR in the host to /CONTAINER-DIR in the Docker
          container. If 'HOST-DIR' is omitted,  Docker automatically creates
       the new
          volume on the host.  The OPTIONS are a comma delimited list and can
       be:

              · [rw|ro]

              · [z|Z]

              · [[r]shared|[r]slave|[r]private]

              · [delegated|cached|consistent]

              · [nocopy]

       The CONTAINER-DIR must be an absolute path such as /src/docs. The
       HOST-DIR can be an absolute path or a name value. A name value must
       start with an alphanumeric character, followed by a-z0-9, _
       (underscore), . (period) or - (hyphen). An absolute path starts with a
       / (forward slash).

       If you supply a HOST-DIR that is an absolute path,  Docker bind-mounts
       to the path you specify. If you supply a name, Docker creates a named
       volume by that name. For example, you can specify either /foo or foo
       for a HOST-DIR value. If you supply the /foo value, Docker creates a
       bind mount. If you supply the foo specification, Docker creates a named
       volume.

       You can specify multiple  -v options to mount one or more mounts to a
       container. To use these same mounts in other containers, specify the
       --volumes-from option also.

       You can supply additional options for each bind mount following an
       additional colon.  A :ro or :rw suffix mounts a volume in read-only or
       read-write mode, respectively. By default, volumes are mounted in
       read-write mode.  You can also specify the consistency requirement for
       the mount, either :consistent (the default), :cached, or :delegated.
       Multiple options are separated by commas, e.g. :ro,cached.

       Labeling systems like SELinux require that proper labels are placed on
       volume content mounted into a container. Without a label, the security
       system might prevent the processes running inside the container from
       using the content. By default, Docker does not change the labels set by
       the OS.

       To change a label in the container context, you can add either of two
       suffixes :z or :Z to the volume mount. These suffixes tell Docker to
       relabel file objects on the shared volumes. The z option tells Docker
       that two containers share the volume content. As a result, Docker
       labels the content with a shared content label. Shared volume labels
       allow all containers to read/write content.  The Z option tells Docker
       to label the content with a private unshared label.  Only the current
       container can use a private volume.

       By default bind mounted volumes are private. That means any mounts done
       inside container will not be visible on host and vice-a-versa. One can
       change this behavior by specifying a volume mount propagation property.
       Making a volume shared mounts done under that volume inside container
       will be visible on host and vice-a-versa. Making a volume slave enables
       only one way mount propagation and that is mounts done on host under
       that volume will be visible inside container but not the other way
       around.

       To control mount propagation property of volume one can use :[r]shared,
       :[r]slave or :[r]private propagation flag. Propagation property can be
       specified only for bind mounted volumes and not for internal volumes or
       named volumes. For mount propagation to work source mount point (mount
       point where source dir is mounted on) has to have right propagation
       properties. For shared volumes, source mount point has to be shared.
       And for slave volumes, source mount has to be either shared or slave.

       Use df <source-dir> to figure out the source mount and then use findmnt
       -o TARGET,PROPAGATION <source-mount-dir> to figure out propagation
       properties of source mount. If findmnt utility is not available, then
       one can look at mount entry for source mount point in
       /proc/self/mountinfo. Look at optional fields and see if any
       propagation properties are specified.  shared:X means mount is shared,
       master:X means mount is slave and if nothing is there that means mount
       is private.

       To change propagation properties of a mount point use mount command.
       For example, if one wants to bind mount source directory /foo one can
       do mount --bind /foo /foo and mount --make-private --make-shared /foo.
       This will convert /foo into a shared mount point. Alternatively one can
       directly change propagation properties of source mount. Say / is source
       mount for /foo, then use mount --make-shared / to convert / into a
       shared mount.

              Note: When using systemd to manage the Docker daemon's start and
              stop, in the systemd unit file there is an option to control
              mount propagation for the Docker daemon itself, called
              MountFlags. The value of this setting may cause Docker to not
              see mount propagation changes made on the mount point. For
              example, if this value is slave, you may not be able to use the
              shared or rshared propagation on a volume.

       To disable automatic copying of data from the container path to the
       volume, use the nocopy flag. The nocopy flag can be set on bind mounts
       and named volumes.

       See also --mount, which is the successor of --tmpfs and --volume.  Even
       though there is no plan to deprecate --volume, usage of --mount is
       recommended.

Docker Community              Docker User Manuals                    DOCKER(1)