# OverlayFS

> overlaying Linux filesystem

**Wikidata**: [Q18215953](https://www.wikidata.org/wiki/Q18215953)  
**Wikipedia**: [English](https://en.wikipedia.org/wiki/OverlayFS)  
**Source**: https://4ort.xyz/entity/overlayfs

## Summary  
OverlayFS is a Linux kernel file system that lets one directory be overlaid on top of another, presenting a single unified view of multiple layers. It was initially contributed by Hungarian developer Miklos Szeredi in 2010 and is now a standard part of Linux distributions.

## Key Facts  
- **Developer:** Miklos Szeredi submitted the initial RFC patchset for OverlayFS in 2010【developer】.  
- **Type:** It is classified as a *file system* (instance of “file system”)【instance_of】.  
- **Operating System:** OverlayFS runs on the Linux kernel【operating_system】.  
- **Official Documentation:** The kernel’s upstream documentation is hosted at https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html【website】.  
- **Wikidata ID:** /m/0121r8cp (freebase_id)【freebase_id】.  
- **Wikipedia Presence:** The article “OverlayFS” exists in 7 languages, including English, Czech, German, French, Norwegian, Ukrainian, and Chinese【wikipedia_languages】.  
- **Arch Wiki Articles:** English, Spanish, and Russian Arch Wiki pages cover OverlayFS【archwiki_article】.  
- **Gentoo Wiki Articles:** Available in English, French, Hungarian, and Japanese【gentoo_wiki_article】.  
- **Sitelink Count:** The Wikidata entry links to 7 external sites【sitelink_count】.

## FAQs  
### Q: What is OverlayFS?  
A: OverlayFS is a Linux file system that merges a lower (read‑only) directory with an upper (writable) directory, exposing their combined contents as a single directory tree.  

### Q: Who created OverlayFS and when?  
A: The initial RFC patchset for OverlayFS was submitted by Hungarian programmer Miklos Szeredi in 2010.  

### Q: Is OverlayFS part of the main Linux kernel?  
A: Yes, OverlayFS is integrated into the Linux kernel and is documented in the official kernel filesystems guide.  

### Q: Where can I find official documentation for OverlayFS?  
A: The authoritative reference is the kernel documentation page at https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html.  

### Q: Which operating systems support OverlayFS?  
A: OverlayFS is designed for and supported on Linux‑based systems.  

## Why It Matters  
OverlayFS provides a lightweight, kernel‑level mechanism for combining multiple directory trees into a single coherent view. By allowing a writable layer to sit atop a read‑only base, it enables use cases such as applying temporary changes, building layered container images, and performing live system updates without altering the underlying data. Because it is built into the Linux kernel, it offers high performance and broad compatibility across distributions, making it a foundational tool for developers and system administrators who need flexible, non‑intrusive file‑system layering.

## Notable For  
- **Kernel Integration:** Delivered as a native Linux kernel file system, avoiding the need for external modules.  
- **Creator Reputation:** Originated from Miklos Szeredi, a well‑known contributor to Linux file‑system technology.  
- **Cross‑Language Documentation:** Covered on major wiki platforms (Arch, Gentoo) in multiple languages, reflecting wide community adoption.  
- **Broad OS Support:** Explicitly targeted at Linux, ensuring consistent behavior across all major distributions.  
- **Minimal Overhead:** Implements overlaying directly in the VFS layer, providing efficient read/write operations without extra user‑space components.  

## Body  

### Overview  
OverlayFS merges two (or more) directories—commonly called the *lower* (read‑only) and *upper* (writable) layers—into a single mount point. The resulting view shows files from both layers, with the upper layer taking precedence when filenames clash.

### Development History  
- **2010:** Miklos Szeredi submitted the first RFC patchset, introducing the concept to the kernel community.  
- Subsequent revisions were merged into the mainline kernel, where the file system is now maintained as part of the core filesystems suite.

### Technical Characteristics  
- **Layer Model:** Supports at least one lower and one upper layer; additional lower layers can be stacked.  
- **Copy‑on‑Write:** When a file from the lower layer is modified, it is copied to the upper layer before the write occurs.  
- **Namespace Compatibility:** Works with standard Linux mount namespaces, allowing per‑namespace overlay configurations.  

### Integration with Linux  
- **Kernel Documentation:** Detailed usage, mount options, and limitations are described in the kernel’s official OverlayFS guide.  
- **Distribution Support:** Documented on Arch, Gentoo, and other distro wikis, indicating ready‑to‑use packages and configuration examples.  

### References & Further Reading  
- Official kernel documentation: https://www.kernel.org/doc/html/latest/filesystems/overlayfs.html  
- Wikipedia entry “OverlayFS” (available in multiple languages).  
- Arch Wiki pages “Overlay_filesystem” (English, Spanish, Russian).  
- Gentoo Wiki pages “OverlayFS” (English, French, Hungarian, Japanese).