Docker Storage: Volume mounts, Bind mounts, Tmpfs mounts.

What is Docker?

Docker is a software platform for building applications based on containers — small and lightweight execution environments that make shared use of the operating system kernel but otherwise run in isolation from one another. While containers have been used in Linux and Unix systems for some time, Docker, an open-source project launched in 2013, helped popularize the technology by making it easier than ever for developers to package their software to “build once and run anywhere.”

Docker Storage mounts:

Containers don’t write data permanently to any storage location. Docker storage must be configured if you would like your container to store data permanently. The data doesn’t prevail when the container is deleted (using the remove command); this happens because when the container is deleted, the writable layer is also deleted. If the data is stored outside the container you can use it even if the container no longer exists.

If a container crashes and can’t be restored/restarted the data is gone! But, normally containers can be restarted and continued — in that case, the data is not lost. So, it’s always advisable moreover mandatory to mount the data outside the container.
For instance, the following sequence from the docker docs illustrates how container startups work. Note that the data is not lost here until the container is removed.

Docker storage distinguishes three storage types. Two types are permanent: Docker volumes and bind Mounts and the third way of writing data is tmpfs. From the container perspective, it doesn’t know what sort of storage is in use.

The difference between these is, that volumes have a dedicated filesystem on the host (/var/lib/ docker/volumes) and are directly controlled through the Docker CLI. On the other hand, bind mounts use any available host filesystem. Whereas tmfs, uses the host memory.

  1. Docker Volume mount:

Docker volume is the most commonly used technology for the permanent storage of container data. Docker volume is managed by Docker itself and has a dedicated filesystem on the host, doesn’t depend upon the filesystem structure on the host. Docker volumes are explicitly managed via the Docker command line and can be created alone or during container initialization. The command used is docker volume create.

When stopping or deleting a container, the Docker volume remains permanently stored. The volumes are often manually deleted with the docker volume prune command. Multiple containers can be connected to the same Docker volume.

2. Docker Bind mount:

Docker bind mount is the second permanent storage option but with more limited options than Docker volume. It can’t be managed via Docker CLI and is totally dependent on the availability of the filesystem of the host. A host filesystem can be created when running a container. Bind mounts are a sort of superset of Volumes (named or unnamed).

3. Docker Tmpfs mount:

tmpfs is a third storage option that is not permanent like Docker volume or bind mount. The data is written directly on to the host’s memory and deleted when the container is stopped. Very useful when it involves sensitive data that you simply don’t want to be permanent. A really significant difference is that containers can’t share tmpfs space unless they’re running on Linux OS. Two flags are used when creating tmpfs volume: tmpfs and mount. Mount flag is newer and supports multiple options during container startup. Temporary filesystems are written to RAM (or to your swap file if RAM is filling up) and not to the host or the container’s own filesystem layer at Docker tmpfs.

So I hope this was very helpful and if you have any doubts or any questions you can write them down in the comment section below and I will try to answer you as soon as I can.

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