Each file system can have its own properties and permissions, and can hide or override files from the lower file systems. UFS allows multiple file systems to be mounted on top of each other, forming a single coherent file system. This reduces the bandwidth and storage requirements for downloading images.ĭocker images are based on a concept called union file systems ( UFS). When you pull an image from a registry (such as Docker Hub), you only download the layers that you don’t already have on your disk.

Similarly, if you have two containers that are based on the same image, you only need to store the image layers once on your disk.Īnother benefit of using layers is that they enable fast and efficient delivery of images. For example, if you have two images that are based on the same base layer, you only need to store the base layer once on your disk. The advantage of using layers is that they allow reuse and sharing of common components among different images and containers. The container layer is deleted when the container is removed. This layer is called the container layer, and it contains any changes made to the container during its lifetime. However, when a container is created from an image, a new writable layer is added on top of the image layers. The layers in a Docker image are read-only, which means they cannot be modified once they are created. The base layer is the lowest layer in the image hierarchy, and it usually contains the operating system or a minimal runtime environment. Each layer also has a parent layer, except for the base layer, which has no parent. The metadata provides information about the image and how to use it.Įach layer is identified by a unique ID or a digest, which is a hash of the layer’s content. For example, a layer can be created by installing a package, copying a file, or running a command.Ī Docker image also has metadata, such as the image name, tags, labels, environment variables, exposed ports, volumes, entrypoint, and command. A Docker image consists of multiple layers, each layer representing a change or an addition to the image. What are Docker images?Ī Docker image is a file that contains all the information and instructions to create and run a container. The image layers, on the other hand, are persistent and immutable, meaning that they are not modified or deleted unless you explicitly do so. The container layer is ephemeral, meaning that it is discarded when the container is stopped or deleted.

This layer stores any changes that are made to the container during its lifetime, such as creating, modifying, or deleting files. When you run a Docker container from an image, Docker creates a thin writable layer on top of the image layers, called the container layer.