What is a union filesystem in the context of container images?

What is a union filesystem in the context of container images?

• A. A memory management technique
• B. A network overlay for containers
• C. A usability feature of container names
• D. A filesystem that combines individual layers into a single view

Answer: (D)

A union filesystem is a system that presents multiple directories as one unified filesystem. In container images, each image is built from layered changes. A runtime uses a union filesystem (such as OverlayFS or AUFS) to stack these layers so the container appears to have a single, coherent filesystem. The lower layers are read-only, and there is a writable top layer where any changes are recorded. When a file is modified, the write goes to this top layer without altering the underlying layers, a copy-on-write behavior. This arrangement keeps storage efficient, lets many containers share common base layers, and provides a clean, modular way to compose images from multiple layers.

A union filesystem is a system that presents multiple directories as one unified filesystem. In container images, each image is built from layered changes. A runtime uses a union filesystem (such as OverlayFS or AUFS) to stack these layers so the container appears to have a single, coherent filesystem. The lower layers are read-only, and there is a writable top layer where any changes are recorded. When a file is modified, the write goes to this top layer without altering the underlying layers, a copy-on-write behavior. This arrangement keeps storage efficient, lets many containers share common base layers, and provides a clean, modular way to compose images from multiple layers.