|Developer(s)||Open Source Community|
|Full name||Unification File System|
|File system permissions||POSIX|
|Transparent encryption||No (but can be provided at the block device level)|
|Supported operating systems||Linux, FreeBSD and NetBSD|
Unionfs is a filesystem service for Linux, FreeBSD and NetBSD which 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.
When mounting branches, the priority of one branch over the other is specified. So when both branches contain a file with the same name, one gets priority over the other.
The different branches may be either read-only or read-write file systems, so that writes to the virtual, merged copy are directed to a specific real file system. 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. This may be desirable when the media is physically read-only, such as in the case of Live CDs.
In Knoppix, a union between the file system on the CD-ROM or DVD and a file system contained in an image file called knoppix.img (knoppix-data.img for Knoppix 7) on a writable drive (such as a USB memory stick) can be made, where the writable drive has priority over the read-only filesystem. This allows the user to change any of the files on the system, with the new file stored in the image and transparently used instead of the one on the CD.
Docker uses Unionfs to layer Docker images. As actions are done to a base image, layers are created and documented, such that each layer fully describes how to recreate an action. This strategy enables Docker's lightweight images, as only layer updates need to be propagated (compared to full VMs, for example).
UbuntuLTSP, the Linux Terminal Server Project implementation for Ubuntu, uses Unionfs when PXE booting thin or thick clients.
Unionfs for Linux has two versions. Version 1.x is a standalone one that can be built as a module. Version 2.x is a newer, redesigned, and reimplemented one.
overlayfs written by Miklos Szeredi has been used in OpenWRT and considered by Ubuntu and has been merged into the mainline Linux kernel on 26 October 2014  after many years of development and discussion for version 3.18 of the kernel.
unionfs-fuse is an independent project, implemented as a user space filesystem program, instead of a kernel module or patch. Like Unionfs, it supports copy-on-write and read-only or read-write branches.
mhddfs works like Unionfs but permits balancing files over drives with the most free space available. It is implemented as a user space filesystem.
mergerfs is a FUSE based union filesystem which offers multiple policies for accessing and writing files as well as other advanced features (xattrs, managing mixed RO and RW drives, link CoW, etc.).
Sun Microsystems introduced the first implementation of a stacked, layered file system with copy-on-write, whiteouts (hiding files in lower layers from higher layers), etc. as the Translucent File Service in SunOS 3, circa 1986.
- Wright, Charles P.; Zadok, Erez (December 2004). "Kernel Korner: Unionfs: Bringing Filesystems Together". Linux Journal. 2004 (128): 8. ISSN 1075-3583.
- "Git server of File systems and Storage Lab". Stony Brook University. Archived from the original on 17 June 2016. Retrieved 17 July 2017.
Project: unionfs-3.9.y.git; Owner: Erez Zadok
- "File systems and Storage Lab: CS Department". Stony Brook University. Retrieved 2017-07-17.
Lab Coordinator and Principal Investigator: Prof. Erez Zadok
- Schulz, Werner P. (2016-12-15). "Hard drive Installation: UNIONFS". Knoppix. Retrieved 2017-07-17.
- "Docker overview: Union file systems". Docker, Inc. 2017-07-13. Retrieved 2017-07-17.
- Georgopoulos, Alkis (2010-12-24). "UbuntuLTSP/LTSPWithoutNFS". Retrieved 2017-07-17.
- "aufs Homepage". 2017-07-02. Retrieved 2017-07-17.
- Torvalds, Linus (2014-10-26). "overlayfs merge + leak fix for d_splice_alias() failure exits". Retrieved 2017-07-17.
- Corbet, Jonathan (2011-06-15). "Debating overlayfs". Retrieved 2017-07-17.
- Podgorny, Radek (2016-11-21). "unionfs-fuse - A userspace unionfs implementation". Retrieved 2017-07-17.
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- "trapexit/mergerfs". GitHub. Retrieved 2018-10-21.
- Sun/Release 3.0. vol. 1. Interleaf Technical Publishing. 1988. pp. 15‑1–18, 16‑1–19.
- Imran, Awais (2011-06-19). "Comex Working On Faster Jailbreaking Process for JailbreakMe 3.0; iPad 2 Jailbreak Coming Soon!". Redmond Pie. Retrieved 2017-07-17.
- Official website
- Charles P. Wright, Jay Dave, Puja Gupta, Harikesavan Krishnan, Erez Zadok and Mohammad Nayyer Zubair (2004). "Versatility and Unix Semantics in a Fan-Out Unification File System" (PDF). Technical Report FSL-04-01b. Stony Brook University.
- unionfs-fuse on GitHub - A FUSE-based alternative implementation of Unionfs
- FunionFS - Another FUSE-based implementation of Unionfs
- The new unionfs implementation for FreeBSD and status of merging (2007-10-23)
- On Incremental File System Development
- LUFS-based unionfs for Linux (based on LUFS)
- DENX U-Boot and Linux Guide: Overlay File Systems