# UnionFS

> filesystem service for Linux

**Wikidata**: [Q1886171](https://www.wikidata.org/wiki/Q1886171)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/UnionFS)  
**Source**: https://4ort.xyz/entity/unionfs

## Summary
UnionFS is a filesystem service for Linux that allows multiple directories to be transparently overlaid into a single virtual filesystem. It enables the combination of read-only and read-write directories, making it useful for live CD distributions and container environments.

## Key Facts
- Instance of: file system
- Website: http://unionfs.filesystems.org
- Wikipedia title: UnionFS
- Freebase ID: /m/027x48
- Stack Exchange tag: https://stackoverflow.com/tags/unionfs
- Wikipedia languages: ar, de, en, es, fa, fr, it, ja, nl, no
- Arch Linux package: unionfs-fuse
- Operating systems: Linux, Berkeley Software Distribution, FreeBSD
- Sitelink count: 12

## FAQs
### Q: What is UnionFS used for?
A: UnionFS is used to overlay multiple directories into a single virtual filesystem, allowing read-only and read-write directories to be combined. This is particularly useful for live CD distributions, container environments, and scenarios where you need to merge multiple filesystem layers.

### Q: Which operating systems support UnionFS?
A: UnionFS runs on Linux, Berkeley Software Distribution (BSD), and FreeBSD operating systems.

### Q: Is UnionFS free software?
A: Yes, UnionFS is free software, as indicated by its availability on FreeBSD (a free Unix-like operating system) and its presence in Arch Linux's package repositories.

## Why It Matters
UnionFS solves a fundamental problem in computing: how to efficiently combine multiple filesystem layers without duplicating data. By creating a virtual filesystem that overlays multiple directories, it enables powerful use cases like live CD distributions where a read-only base system can be combined with a writable layer for user changes. This technology is foundational to modern containerization and virtualization approaches, where multiple filesystem layers need to be merged transparently. The ability to stack filesystems also enables efficient snapshotting and rollback capabilities, making UnionFS valuable for system administration, software testing, and deployment scenarios where isolation and flexibility are critical.

## Notable For
- Transparently overlays multiple directories into a single virtual filesystem
- Enables read-only and read-write directory combinations
- Supports multiple operating systems including Linux, BSD, and FreeBSD
- Used in live CD distributions and container technologies
- Provides efficient filesystem layering without data duplication

## Body
### Technical Architecture
UnionFS operates by creating a virtual filesystem that merges the contents of multiple directories. When a file is accessed, UnionFS searches through the stacked directories in order and returns the first matching file it finds. This creates a unified view of multiple filesystem layers.

### Implementation Details
The filesystem uses a stackable unification mechanism where directories are layered on top of each other. Lower layers are typically read-only while upper layers can be read-write, allowing changes to be made without modifying the underlying data. This is particularly useful for creating writable environments on top of read-only media.

### Use Cases
UnionFS is commonly used in live CD distributions where a read-only base system needs to be combined with a writable layer for user changes. It's also used in container technologies and virtualization platforms where multiple filesystem layers need to be merged. The technology enables efficient snapshotting and rollback capabilities by allowing changes to be made in an upper layer while preserving the original data in lower layers.

### Related Technologies
UnionFS is related to other union-capable filesystems like AUFS (Another Union File System) and overlayfs. While UnionFS was one of the earlier implementations of this concept, newer filesystems have been developed that build upon similar principles but with different performance characteristics and feature sets.

## References

1. Freebase Data Dumps. 2013