AUFS, which stands for "Advanced Multi-Layered Unification Filesystem," is a Linux filesystem that implements union mounting. It allows you to overlay multiple directories (branches) on top of each other, presenting them as a single, unified directory.
Key aspects of AUFS:
Union Mounting: This is the core concept. AUFS combines multiple directory trees into one. Changes made to the unified view are typically directed to a writeable branch.
Branches: These are the individual directories that are being overlaid. One or more can be read-only, while typically only one is writeable. The order of the branches is important.
Writeable Branch: Typically, changes (new files, modifications, deletions) are directed to the top-most, writeable branch. If a file exists in a lower branch, writing to it will create a copy-on-write (CoW) of that file in the writeable branch.
Copy-on-Write (CoW): When you modify a file that exists in a read-only branch, AUFS copies the file to the writeable branch before making the changes. This ensures the original file in the read-only branch remains unchanged.
Applications: AUFS is commonly used in live CDs/DVDs, Docker/container technologies, and other scenarios where you need to create a layered filesystem. It's useful for creating immutable base images with mutable layers on top.
Relationship with Docker/Containers: Older versions of Docker often used AUFS as a storage driver. This enabled efficient layer sharing among containers. While AUFS is still usable, other storage drivers like OverlayFS are more commonly used now.
Alternatives: OverlayFS is a newer, simpler, and often faster union filesystem that is now the preferred choice in many situations. Other alternatives include UnionFS.
Kernel Integration: AUFS was never fully merged into the mainline Linux kernel due to complexity and licensing concerns. It typically requires a separate patch. OverlayFS, on the other hand, is part of the mainline kernel.
Here are some important subjects related to AUFS:
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