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 and 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.
Unionfs was originally developed by Professor Erez Zadok and his team at
Stony Brook University. Uses
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 on a writable drive 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. Unionfs can also be used to create a
single common template for a number of file systems, or for security reasons.
It is sometimes used as an ad hoc snapshotting system.
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.
Other implementations 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. Version 2.x is the smallest union
file system implementation for Linux, it is heavily tested and inspected by many
kernel developers, and it is more efficient.
aufs is an alternative version of unionfs.
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. Plan 9 from Bell Labs operating system
uses union mounts extensively to build custom namespaces per user or processes.
Union mounts have also been available in BSD since at least 1995.
The GNU Hurd has an implementation of Unionfs. As of January 2008, it works,
but results in a read-only mount-point. mhddfs works like Unionfs but permits
balancing files over drives with the most free space available. It is
implemented as a user space filesystem. Sun Microsystems introduced the first
implementation of a stacked, layered file system with copy-on-write,
whiteouts, etc. as the Translucent File Service in SunOS 3, circa 1986.
JailbreakMe 3.0, a tool for jailbreaking iOS devices released in July 2011, uses
unionfs techniques to speed up the installation process of the operating
system modification. UbuntuLTSP, the Linux Terminal Server
Project implementation for Ubuntu, uses Unionfs when PXE booting thin or thick
clients. References
C. P. Wright et al.. "Versatility and Unix Semantics in a Fan-Out Unification
File System." External links
Official Unionfs home page unionfs-fuse FUSE-based alternative
implementation of Unionfs aufs is an alternative union file system
for Linux. FunionFS
Kernel Korner - Unionfs: Bringing Filesystems Together - article from
LinuxJournal The new implementation of union fs for
FreeBSD On implementation of a union fs
LUFS-based unionfs Linux Implementations - excellent
write-up on history of union file systems for Linux
The Translucent File Service, David Hendricks, Sun Microsystems
mhddfs mini_fo: overlay filesystem for
read-only rootfs FreeBSD mount_union man page