Unionfs: A Stackable Unification File System

Unionfs logo    This project builds a stackable unification file system, which can appear to merge the contents of several directories (branches), while keeping their physical content separate. Unionfs is useful for unified source tree management, merged contents of split CD-ROM, merged separate software package directories, data grids, and more. Unionfs allows any mix of read-only and read-write branches, as well as insertion and deletion of branches anywhere in the fan-out. To maintain unix semantics, Unionfs handles elimination of duplicates, partial-error conditions, and more. Unionfs is part of the larger FiST project which includes the Wrapfs wrapper stackable file system.

Software Download:

Unionfs releases of kernel code and user-level utilities, as well as snapshots, are available via FTP or HTTP. Full kernel versions patched with Unionfs can be accessed via Git from our git server.

Download Unionfs 2.x with experimental On Disk Format (ODF) support. Read the ODF Design Document.

Unionfs 2.x is maintained in a Git repository. Here are instructions how to check out the latest code with Git.

See list of mirror sites.

Kernel Version Compatibility:

Mailing Lists:

Click on the list name below to access the subscription Web page. That page will also tell you how to post to the list, access the lists' archives, change your subscription status, etc.

List Name Purpose Who Can Subscribe Who Can Post Comments
unionfs For users' and developers' questions, announcements, bug reports, and general. Anyone Subscribers only  
unionfs-cvs For CVS commit logs Anyone No one: automated CVS logs only  


If you find a problem, please describe it in detail and submit a bug report via Bugzilla.

Some Unionfs users congregate in #unionfs on the FreeNode IRC network.

Users of Real Time (RT) Linux can find additional RT patches for Unionfs.

Various articles and tutorials involving Unionfs:

Projects Using Unionfs:

1ALT LinuxALT Linux distribution build kit, that allow you to preparing customized distribution or LiveCD, based on ALT Linux Sisyphus packages repository
2ArchieArch Live Project
3Ark Linux LiveArk Linux Live is a "Live CD" version of Ark Linux.
4BlutopiaCluster Life-cycle Management, IBM Research Austin
5ClefAgregClefAgreg, a bootable USB key dedidaced to Mathematics, using Debian GNU/linux
6ClusterixClusterix is a modular Linux live CD based on Morphix, Knoppix and Debian GNU/Linux.
7DanixCzech Knoppix-like distribution
8Debian LinuxThe Debian Linux distribution
9DSS-LiveA system development environment to create Debian-based live Linux systems.
10EliveA Debian-based LiveCD centered around the Enlightenment window manager.
11Foresight LinuxForesight Linux is a Distribution which showcases the latest and greatest version of the GNOME Desktop Environment and Freedesktop.org
12GeLuXSwiss French Linux distribution based on Morphix
13Generations LinuxBoot/LiveCD containing GNU/Linux and KDE
14Gentoo LinuxThe Gentoo Linux distribution
15Gibraltar firewallGibraltar Security Firewall LiveCD
16Gnome LiveA Gnome LiveCD
17GoblinXGoblinX is a live CD based on Slackware, with standardized themes for novice users.
18grmlLiveCD for system administrators and texttool-users
19INSERTA Linux rescue system with NTFS write support and virus scanning that fits on a credit-card size CD.
20KanotixKnoppix-based LiveCD.
21KnoppixBootable CD with a collection of GNU/Linux Software
22LiveBackupA Framework to create bootable live-ISO's from installed Linux systems
23m-distAudio/MIDI LiveCD based on Slackware.
24Mandriva LinuxThe Mandriva Linux distribution
25Mandriva OneLive & Install CD with everything for office work, Internet and multimedia.
26MiniMythA network-boot distribution that runs the MythTV PVR software.
27MorphixModular Knoppix-based LiveCD
29open Networking Routersa set of network appliances based on open-source
30open Thin ClientA comprehensive management solution and operating system for thin clients
31OpenSuSE Linux 10.2The Open-SuSE Linux distribution
32PaiPixA remastered Knoppix with a focus on Scientific applications.
33rBuilder OnlineAn online tool for creating and maintaining tailored Linux distributions including LiveCDs, Installable CDs, VMWare Images, QEMU Images, and other image types from custom package sets.
34redWall FirewallA bootable CD-ROM Firewall.
35Scientific Linux Live SystemLiveCD and LiveDVD of Scientific Linux
36T2A "Distribution Build Kit" that allows you to select various targets for a customized distribution (e.g., router, desktop, or LiveCD).
37TaprobaneA Debian-based LiveCD that is customized for convenience without "dumbing down" the user's experience.
38TuguxPortuguese Linux Distribution
39Ubuntu Linux (Gutsy/Edgy)The Ubuntu Linux distribution
40XarnoppixA Catalan-localized LiveCD focused on education

Journal Articles:

# Title (click for html version) Formats Published In Date Comments
1 On Incremental File System Development PS PDF BibTeX ACM Transactions on Storage (TOS) May 2006  
2 Versatility and Unix Semantics in Namespace Unification PS PDF BibTeX ACM Transactions on Storage (TOS) Feb 2006  
3 Unionfs: Bringing File Systems Together BibTeX Linux Journal Dec 2004  

Conference and Workshop Papers:

# Title (click for html version) Formats Published In Date Comments
1 Kernel Support for Stackable File Systems PS PDF BibTeX 2007 Ottawa Linux Symposium Jun 2007  
2 Making Linux Stacking-Friendly BibTeX First Linux Storage and Filesystem Workshop, held in conjuction with the fifth USENIX Conference on File and Storage Technologies (FAST 2007) Feb 2007  
3 UnionFS: User- and Community-oriented Development of a Unification Filesystem PS PDF BibTeX 2006 Ottawa Linux Symposium Jul 2006  

Technical Reports:

# Title (click for html version) Formats Published In Date Comments
1 Versatility and Unix Semantics in a Fan-Out Unification File System PS PDF BibTeX Stony Brook U. CS TechReport FSL-04-01b Oct 2004  

Past Students:

# Name (click for home page) Program Period Current Location
1 Charles P. Wright PhD May 2003 - May 2006 Application Software Developer, Walleye Software (New York, NY)
2 Shrikar Archak MS Sep 2009 - Dec 2010 Senior Software Engineer, Core Deduplication Filesystem group, Datadomain/EMC
3 Jay Pradip Dave MS May 2003 - Dec 2003 Program Manager, Windows Security, Core Operating System Group, Microsoft (Seattle, WA)
4 Puja Gupta MS Jan 2003 - Dec 2003 Software Engineering Manager, Darwin Runtime, Core OS, Apple Inc. (Cupertino, CA)
5 Rachita Kothiyal MS Sep 2007 - May 2009 Member of Technical Staff, Network Appliance, Inc. (Sunnyvale, CA)
6 Arun Krishnakumar MS Jan 2005 - Dec 2005 Computer Scientist, Acrobat team, Adobe Systems (India)
7 Harikesavan Pathangi Krishnan MS Jan 2003 - Dec 2003 Software Engineer, Engineering department for WAFS product line, Packeteer, Inc. (South Plainfield, NJ)
8 Li Mengyang MS May 2014 - Dec 2014 Member of Technical Staff, Foundation Group, Nutanix (San Jose, CA)
9 Dongju Ok MS Sep 2014 - May 2016  
10 David Quigley MS Sep 2005 - May 2007 ZFS Linux Software Engineer, High Performance Data Division Intel (Longmont, CO)
11 Mohammad Nayyer Zubair MS Jan 2004 - Dec 2004 Software Developer, Bloomberg (New York, NY)
12 Yiannis Pericleous BS/MS Sep 2006 - Jun 2007 Software Developer, Albourne Partners (Nicosia, Cyprus)
13 David Quigley BS Dec 2004 - Aug 2005 ZFS Linux Software Engineer, High Performance Data Division Intel (Longmont, CO)
14 Josef "Jeff" Sipek BS Jul 2005 - May 2008 Nexenta Systems (Ann Arbor, MI)
15 Mohammad Nayyer Zubair BS May 2003 - Dec 2003 Software Developer, Bloomberg (New York, NY)


# Sponsor Amount Period Type Title (click for award abstract)
1 NSF Trusted Computing (TC) $400,000 2003-2006 Sole PI A Layered Approach to Securing Network File Systems

(Last updated: Mon Aug 29 01:30:40 EDT 2016)