Preface
Users can create containers with data volumes through the --volume/-v or --mount options of docker run, but there are some subtle differences between these two options. Here is a summary and combing.
Command Usage
--volume(-v)
Parameters --volume (or abbreviated as -v) can only create bind mounts. Example:
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
Notes:
- Command format: [[HOST-DIR:]CONTAINER-DIR[:OPTIONS]]]
- If HOST-DIR is specified, it must be an absolute path, and will be automatically created if the path does not exist
- rw in the example is read and write, ro is read only
--mount
Parameters --mount are used to mount volume by default, but can also be used to create bind mount and tmpfs. If the type option is not specified, the default is to mount volume. volume is a more flexible data management method, and volume can be managed through the docker volume command set. Example:
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
Notes:
- Mount volume command format: [type=volume,]source=my-volume,destination=/path/in/container[,...]
- Create bind mount command format: type=bind,source=/path/on/host,destination=/path/in/container[,...]
- If you create a bind mount and specify a source, it must be an absolute path and the path must already exist
- In the example readonly means read only
Summary of differences
Comparison between creating bind mount and mount volume
| Comparison items | bind mount | volume |
|---|---|---|
| Source Location | User-specified | /var/lib/docker/volumes/ |
| Source is empty | Overwrite dest is empty | Keep dest content |
| Source is not empty | Overwrite dest content | Overwrite dest content |
| Source Type | File or directory | Only a directory |
| portability | General (self-maintenance) | Strong (docker hosting) |
| Direct access to the host | Easy (just chown) | Restricted (need to log in to root user)* |
*Note: Docker cannot simply use sudo chown someuser: -R /var/lib/docker/volumes/somevolume to open volume content to ordinary users on the host. If more permissions are opened, there is a security risk. In this regard, the Podman design is much more ideal. Volume is stored in the $HOME/.local/share/containers/storage/volumes/ path, which provides convenience and security. Running containers without root permissions is one of the advantages of Podman, and it does benefit a lot during actual use.
Comparison of --volume and --mount when creating bind mount
| Comparison items |
--volumeor-v
|
--mount type=bind |
|---|---|---|
| If the host path does not exist | Automatically create | The command error was reported |
Official Documentation
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. . 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. (. 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, . :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)
This is the article about the use of the Docker volume mount command volume(-v) and mount. For more related contents of Docker volume(-v) and mount, please search for my previous articles or continue browsing the related articles below. I hope everyone will support me in the future!