filesystems - Why does Docker need a Union File System

Question

What exactly does Docker do with Union File system (like AUFS) to create the containers ? If Docker had to use a regular file system instead of a union file system what will be the disadvantages ?

I am looking for specific technical details/internals and not a high level answer.

Answer 1

It is used to:

• avoid duplicating a complete set of files each time you run an image as a new container
• isolate changes to a container filesystem in its own layer, allowing for that same container to be restarted from a known content (since the layer with the changes will have been dismissed when the container is removed)

That UnionFS:

implements a union mount for other file systems. It allows files and directories of separate file systems, known as branches, to be transparently overlaid, forming a single coherent file system.
Contents of directories which have the same path within the merged branches will be seen together in a single merged directory, within the new, virtual filesystem.

This allows a file system to appear as writable, but without actually allowing writes to change the file system, also known as copy-on-write

If you didn't have UnionFS, an 200MB image run 5 times as 5 separates containers would mean 1GB of disk space.

See more at "How does a Docker image work?".
For more technical details, see:

• "Docker storage drivers"
• "Anatomy of a Container: Namespaces, cgroups & Some Filesystem Magic" from Jér?me Petazzoni (jpetazzo).