Docker has revolutionized application development and deployment with its containerization technology. A common question for newcomers (and even experienced users) is: where does Docker actually store those images on the host machine? This tutorial will demystify the location and organization of Docker images, providing a clear understanding of how Docker manages storage.
Docker Storage Basics
Docker images aren’t stored as a single monolithic file. Instead, Docker employs a layered file system. Each instruction in a Dockerfile creates a new layer. These layers are stacked on top of each other, and Docker leverages this structure to efficiently share image components and reduce disk space usage. When you build or pull a Docker image, these layers are saved on the host machine.
Location of Docker Images
The exact location of Docker image data depends on the storage driver Docker is configured to use. Docker supports various storage drivers, each with its own way of organizing and storing image data. Let’s examine the most common scenarios:
1. Linux Systems with aufs, overlay, overlay2, btrfs, devicemapper, or zfs
On most Linux distributions, Docker defaults to using a storage driver like aufs, overlay, or overlay2 (the latter two are preferred for performance). The primary storage location is under the /var/lib/docker/ directory. Within this directory, the organization varies based on the driver:
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aufs(Advanced Multi-Layered Unification FS):/var/lib/docker/aufs/diff/<image_id>: This directory contains the actual file contents of the image layers./var/lib/docker/repositories-aufs: A JSON file that stores metadata about the local images, including their names, tags, and IDs. You can view this information using thedocker imagescommand.
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devicemapper:/var/lib/docker/devicemapper/devicemapper/data: Stores the image data./var/lib/docker/devicemapper/devicemapper/metadata: Stores metadata related to the image.- Importantly, these files are sparse, meaning they only consume disk space for the actual data stored within the image, not the full allocated size.
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overlay2: This driver is becoming increasingly common. While the precise layout can be more complex, the data is typically found within/var/lib/docker/overlay2.
2. Docker for Mac & Windows (using virtualization)
The storage mechanism differs significantly on macOS and Windows because Docker runs inside a virtual machine.
- Docker for Mac: Docker images are stored within the virtual machine’s disk image. As of recent versions, the primary disk image file is located at:
~/Library/Containers/com.docker.docker/Data/vms/0/Docker.raw - Docker for Windows (using Hyper-V): Docker images are stored within a virtual hard disk file:
C:\Users\Public\Documents\Hyper-V\Virtual hard disks\MobyLinuxVM.vhdx
You can access this virtual machine through Hyper-V Manager if you need to inspect the storage directly.
Important Considerations
- Storage Driver Selection: The choice of storage driver can significantly impact performance and disk space usage. Consider the specific requirements of your workload when choosing a driver.
- Disk Space Management: Docker images can consume a significant amount of disk space, especially if you frequently build and pull new images. Regularly prune unused images, containers, and volumes to reclaim disk space using commands like
docker system prune. - Image Layers: Understanding image layers is crucial for optimizing image size and build times. Use multi-stage builds and minimize the number of layers to create efficient images.