To Understand Docker Volume, you need to go though below sequence.
Docker File System
Bind Mounts
Docker Volumes
1) Docker File System
A docker container runs the software stack defined in an image. Images are made of a set of read-only layers that work on a file system called the Union File System. When we start a new container, Docker adds a read-write layer on the top of the image layers allowing the container to run as though on a standard Linux file system. So, any file change inside the container creates a working copy in the read-write layer. However, when the container is stopped or deleted, that read-write layer is lost.
2) Bind Mounts
A Docker bind mount is a high-performance connection from the container to a directory on the host machine. It allows the host to share its own file system with the container, which can be made read-only or read-write.
This allows us to use a container to run tools that we don't want to install on our host, and yet still work with our host's files. For example, if we wanted to use a custom version of bash for a particular script, we might execute that script in a bash container, mounted to our current working directory:
e.g.,
$ docker run -v $(pwd):/var/opt/project bash:latest \
bash -c "echo Hello > /var/opt/project/file.txt"
The –v option can be used for all forms of mounting and specifies, in this instance, the source on the host – the working directory in the output of $(pwd) – and the target mount point in the container – /var/opt/project.
After running this command, we will find file.txt in the working directory of our host machine. This is a simple way to provide persistent files between invocations of a Docker container, though it is most useful for when the container is doing work on behalf of the host.
One good use case for this would be executing various versions of a language's build tools in Docker to avoid having conflicting installations on a developer machine.
We should note that $(pwd -W) is sometimes needed on Windows bash shells to provide the working directory in a form that the bash shell can pass to Docker.
3) Docker VolumesA bind mount uses the host file system, but Docker volumes are native to Docker. Volumes are the preferred mechanism for persisting data generated by and used by Docker containers. While bind mounts are dependent on the directory structure and OS of the host machine, volumes are completely managed by Docker. Volumes have several advantages over bind mounts:
- Volumes are easier to back up or migrate than bind mounts.
- You can manage volumes using Docker CLI commands or the Docker API.
- Volumes work on both Linux and Windows containers.
- Volumes can be more safely shared among multiple containers.
- Volume drivers let you store volumes on remote hosts or cloud providers, to encrypt the contents of volumes, or to add other functionality.
New volumes can have their content pre-populated by a container.
Volumes on Docker Desktop have much higher performance than bind mounts from Mac and Windows hosts.
In addition, volumes are often a better choice than persisting data in a container’s writable layer, because a volume does not increase the size of the containers using it, and the volume’s contents exist outside the lifecycle of a given container.
Let us do the following scenarios:
- Creating Volumes
- Listing Volumes
- Inspecting Volumes
- Removing Volumes
- Pruning Volumes
- Starting a Container with a Volume
Docker volumes nothing but directory in container and map with base machine. Like it is a network drive which mounted with your base machine.
e.g.,
Assume, you want to copy some files from base machine to containers and files from containers to base machine with regular interval/ daily basis, then docker volume can help you to fullfill this requirement. i.e. both way file movement. Even we can map container to container or container to base machine.
Note:
You need to declared the volume while lunchig the container.
for existing container, we can't declare a volume
Even if a container in stopped state, still we can access volumes
Mapping a volume in two ways:
1) Creating a volume between host to container
2) Creating a volume between container to container
[root@ip-172-31-38-12 ~]# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
ubuntu latest 58db3edaf2be 4 weeks ago 77.8MB
[root@ip-172-31-38-12 ~]#
[root@ip-172-31-38-12 ~]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
[root@ip-172-31-38-12 ~]#
[root@ip-172-31-38-12 docker_class]# ls
dockerfile
-- create Docker file like below
[root@ip-172-31-38-12 docker_class]# cat Dockerfile
FROM ubuntu
MAINTAINER gmohapat
VOLUME /data
-- Create image from dockerfile
[root@ip-172-31-38-12 docker_class]# docker build -t demoimagevol .
Sending build context to Docker daemon 2.048kB
Step 1/3 : FROM ubuntu
---> 58db3edaf2be
Step 2/3 : MAINTAINER gmohapat
---> Running in 7ad715c26e23
Removing intermediate container 7ad715c26e23
---> 54317c9383d4
Step 3/3 : VOLUME /data
---> Running in 123e7f1c699f
Removing intermediate container 123e7f1c699f
---> 44787e8aa6c0
Successfully built 44787e8aa6c0
Successfully tagged demoimagevol:latest
[root@ip-172-31-38-12 docker_class]#
[root@ip-172-31-38-12 docker_class]# docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
demoimagevol latest 44787e8aa6c0 55 seconds ago 77.8MB
ubuntu latest 58db3edaf2be 4 weeks ago 77.8MB
Now we are good to lunch a container
[root@ip-172-31-38-12 docker_class]# docker run -it --name container9 demoimagevol /bin/bash
root@5a158a8f3ce2:/# ls
bin boot data dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@5a158a8f3ce2:/#
Here you see, data volume is there. This 'data' directory is not normal directory. It is OS docker directory.
Let us go inside data volume and create some files and test below.
root@5a158a8f3ce2:/data# touch file1 file2 file3
root@5a158a8f3ce2:/data# ls
file1 file2 file3
Now, come-out from container with exit option.
root@5a158a8f3ce2:/data# exit
exit
[root@ip-172-31-38-12 docker_class]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
5a158a8f3ce2 demoimagevol "/bin/bash" 4 minutes ago Exited (0) 4 seconds ago container9
[root@ip-172-31-38-12 docker_class]#
Now, Create a new contaier and map 'data' volume to new container
[root@ip-172-31-38-12 docker_class]# docker run -it --name container91 --privileged=true --volumes-from container9 ubuntu /bin/bash
root@204b1b533717:/# ls
bin boot data dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@204b1b533717:/# cd data/
root@204b1b533717:/data# ls
file1 file2 file3
root@204b1b533717:/data#
Here, you can see, data volume mapped from previous container and all there files are there in data volume.
Now, do manage some files and check that is reflected in container9.
[root@ip-172-31-38-12 docker_class]# docker ps -a
CONTAINER ID IMAGE COMMAND CREATED STATUS PORTS NAMES
204b1b533717 ubuntu "/bin/bash" 2 minutes ago Exited (0) 14 seconds ago container91
5a158a8f3ce2 demoimagevol "/bin/bash" 15 minutes ago Exited (0) 10 minutes ago container9
[root@ip-172-31-38-12 docker_class]# docker start container9
container9
[root@ip-172-31-38-12 docker_class]# ls
Dockerfile
[root@ip-172-31-38-12 docker_class]# docker attach container9
root@5a158a8f3ce2:/# ls
bin boot data dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@5a158a8f3ce2:/# cd data/
root@5a158a8f3ce2:/data# ls
file1 file2
root@5a158a8f3ce2:/data#
Here you can see, in another container changes reflected.
Declare a volume using command line:
[root@ip-172-31-38-12 docker_class]# docker run -it --name container8 -v /backup ubuntu /bin/bash
root@d0912a3f8083:/# ls
backup bin boot dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@d0912a3f8083:/#
Now map the volume to host machine.
[root@ip-172-31-38-12 docker_class]# docker run -it --name container7 -v /home/ec2-user/newdir:/datavol --privileged=true ubuntu /bin/bash
root@494ef9cc4365:/# ls
bin boot datavol dev etc home lib lib32 lib64 libx32 media mnt opt proc root run sbin srv sys tmp usr var
root@494ef9cc4365:/#
root@494ef9cc4365:/# cd datavol/
root@494ef9cc4365:/datavol# touch samplefile1 samplefile2
root@494ef9cc4365:/datavol# ls
samplefile1 samplefile2
root@494ef9cc4365:/datavol# exit
exit
[root@ip-172-31-38-12 newdir]# cd /home/ec2-user/newdir
[root@ip-172-31-38-12 newdir]# ls -l
total 0
-rw-r--r-- 1 root root 0 Feb 26 19:41 samplefile1
-rw-r--r-- 1 root root 0 Feb 26 19:41 samplefile2
[root@ip-172-31-38-12 newdir]#
Yes, now we can see what files we created in container volume, we able to see in host machine file system.
In real time, can access log files, source files, artifacts, configuration files.
