1. Manage data
By default all files created inside a container are stored on a writable container layer. This means that:
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The data doesn’t persist when that container no longer exists, and it can be difficult to get the data out of the container if another process needs it.
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A container’s writable layer is tightly coupled to the host machine where the container is running. You can’t easily move the data somewhere else.
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Writing into a container’s writable layer requires a storage driver to manage the filesystem. The storage driver provides a union filesystem, using the Linux kernel. This extra abstraction reduces performance as compared to using data volumes, which write directly to the host filesystem.
No matter which type of mount you choose to use, the data looks the same from within the container. It is exposed as either a directory or an individual file in the container’s filesystem.
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Volumes are stored in a part of the host filesystem which is managed by Docker (/var/lib/docker/volumes/ on Linux).
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Non-Docker processes should not modify this part of the filesystem.
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Bind mounts may be stored anywhere on the host system. They may even be important system files or directories.
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Non-Docker processes on the Docker host or a Docker container can modify them at any time.
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tmpfs mounts are stored in the host system’s memory only, and are never written to the host system’s filesystem.
1.1. Volumes
Created and managed by Docker. You can create a volume explicitly using the docker volume create command, or Docker can create a volume during container or service creation.
When you create a volume, it is stored within a directory on the Docker host. When you mount the volume into a container, this directory is what is mounted into the container. This is similar to the way that bind mounts work, except that volumes are managed by Docker and are isolated from the core functionality of the host machine.
A given volume can be mounted into multiple containers simultaneously. When no running container is using a volume, the volume is still available to Docker and is not removed automatically. You can remove unused volumes using docker volume prune.
Volumes also support the use of volume drivers, which allow you to store your data on remote hosts or cloud providers, among other possibilities.
1.1.1. use cases for volumes
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Sharing data among multiple running containers. If you don’t explicitly create it, a volume is created the first time it is mounted into a container. When that container stops or is removed, the volume still exists. Multiple containers can mount the same volume simultaneously, either read-write or read-only. Volumes are only removed when you explicitly remove them.
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When the Docker host is not guaranteed to have a given directory or file structure. Volumes help you decouple the configuration of the Docker host from the container runtime.
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When you want to store your container’s data on a remote host or a cloud provider, rather than locally.
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When you need to back up, restore, or migrate data from one Docker host to another, volumes are a better choice. You can stop containers using the volume, then back up the volume’s directory (such as /var/lib/docker/volumes/<volume-name>).
1.2. Bind mounts
Bind mounts have limited functionality compared to volumes.
When you use a bind mount, a file or directory on the host machine is mounted into a container. The file or directory is referenced by its full path on the host machine.
The file or directory does not need to exist on the Docker host already. It is created on demand if it does not yet exist.
Bind mounts are very performant, but they rely on the host machine’s filesystem having a specific directory structure available.
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Bind mounts allow access to sensitive files
One side effect of using bind mounts, for better or for worse, is that you can change the host filesystem via processes running in a container, including creating, modifying, or deleting important system files or directories. This is a powerful ability which can have security implications, including impacting non-Docker processes on the host system |
1.2.1. use cases for bind mounts
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Sharing configuration files from the host machine to containers. This is how Docker provides DNS resolution to containers by default, by mounting /etc/resolv.conf from the host machine into each container.
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Sharing source code or build artifacts between a development environment on the Docker host and a container. For instance, you may mount a Project target/ directory into a container, and each time you build the Project on the Docker host, the container gets access to the rebuilt artifacts.
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When the file or directory structure of the Docker host is guaranteed to be consistent with the bind mounts the containers require.
1.3. tmpfs mounts
A tmpfs mount is not persisted on disk, either on the Docker host or within a container. It can be used by a container during the lifetime of the container, to store non-persistent state or sensitive information. For instance, internally, swarm services use tmpfs mounts to mount secrets into a service’s containers.
1.3.1. use cases for tmpfs mount
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tmpfs mounts are best used for cases when you do not want the data to persist either on the host machine or within the container. This may be for security reasons or to protect the performance of the container when your application needs to write a large volume of non-persistent state data.
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keep the following in mind
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2. Share data among machines
When building fault-tolerant applications, you need to configure multiple replicas of the same service to have access to the same files.
There are several ways to achieve this when developing your applications.
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One is to add logic to your application to store files on a cloud object storage system like Amazon S3.
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Another is to create volumes with a driver that supports writing files to an external storage system like NFS or Amazon S3.
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Volume drivers
Volume drivers allow you to abstract the underlying storage system from the application logic. |
2.1. External storage system
There are a lot of different systems to use.
We are going to look at the two most poppular systems (they both support any combination of the aforementioned ways).
A more detailed list of systems can be found at the bottom of the page.
2.1.2. IPFS (Inter-Planetary File System)
A peer-to-peer hypermedia protocol, designed to make the web faster, safer, and more open.
2.2. Extend docker
You can extend the capabilities of the Docker Engine by loading third-party plugins.
Docker supports authorization, volume and network driver plugins