Explain overlay as used in linux filesystems, refer to the sample output below: # df -h Filesystem Size Used Avail Use% Mounted on tmpfs 6.3G 1.3M 6.3G 1% /run /dev/mapper/ubuntu--vg-ubuntu--lv 98G 26G 68G 28% / tmpfs 32G 0 32G 0% /dev/shm tmpfs 5.0M 0 5.0M 0% /run/lock /dev/sda2 2.0G 243M 1.6G 14% /boot /dev/sda1 1.1G 6.1M 1.1G 1% /boot/efi tmpfs 6.3G 4.0K 6.3G 1% /run/user/1000 overlay 98G 26G 68G 28% /var/lib/docker/overlay2/9e2396901616c7a13454d3f5a305f4217a22d809b0dd4daeb2f1bab47f7c94c8/merged
The overlay filesystem (OverlayFS) in Linux is a type of union filesystem that allows you to overlay one filesystem on top of another. This is particularly useful in scenarios where you want to combine multiple directory trees into a single, unified view. Here’s a breakdown of how it works: Basic Concepts
- Upper and Lower Layers: OverlayFS uses two main layers:
- Lower Layer: Typically mounted in read-only mode. It contains the base files and directories.
- Upper Layer: Mounted in read-write mode. Any changes made by the user are written here.
- Merged Layer: The combination of the upper and lower layers, which is presented to the user as a single, unified filesystem.
How It Works When you access a file:
- If the file exists in the upper layer, it is used.
- If the file does not exist in the upper layer, the system looks for it in the lower layer.
- If you modify a file from the lower layer, a copy of the file is created in the upper layer (this is called “copy-up”), and the changes are made to this copy.
Example in Docker In your df -h output, the overlay filesystem is used by Docker for its storage driver. The path /var/lib/docker/overlay2/.../merged represents the merged view of the upper and lower layers used by Docker containers. Sample Output Explanation
overlay 98G 26G 68G 28% /var/lib/docker/overlay2/9e2396901616c7a13454d3f5a305f4217a22d809b0dd4daeb2f1bab47f7c94c8/merged
- Size: Total size of the filesystem (98G).
- Used: Space used (26G).
- Avail: Available space (68G).
- Use%: Percentage of space used (28%).
- Mounted on: Mount point of the merged layer.
OverlayFS is widely used in containerization technologies like Docker because it efficiently manages filesystem layers, allowing for quick and flexible container deployments. If you have any more questions or need further clarification, feel free to ask!