computing Computing is any goal-oriented activity requiring, benefiting from, or creating computing machinery. It includes the study and experimentation of algorithmic processes, and development of both hardware and software. Computing has scientific, ...

, OverlayFS is a

union mount In computer operating systems, union mounting is a way of combining multiple directories into one that appears to contain their combined contents. Union mounting is supported in Linux, BSD and several of its successors, and Plan 9, with similar ...

filesystem In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is Computer data storage, stored and retrieved. Without a file system, data placed in a storage me ...

implementation for

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, w ...

. It combines multiple different underlying mount points into one, resulting in single directory structure that contains underlying files and sub-directories from all sources. Common applications overlay a read/write partition over a read-only partition, such as with LiveCDs and IoT devices with limited flash memory write cycles.

History

The need and specification of a kernel mode Linux union mount filesystem was identified in late 2009. The initial RFC patchset of OverlayFS was submitted by Miklos Szeredi in 2010. By 2011,

OpenWrt OpenWrt (from ''open wireless router'') is an open-source project for embedded operating systems based on Linux, primarily used on embedded devices to route network traffic. The main components are Linux, util-linux, musl, and BusyBox. All ...

had already adopted it for their use. It was merged into the

Linux kernel mainline The Linux kernel is a free and open-source, monolithic, modular, multitasking, Unix-like operating system kernel. It was originally authored in 1991 by Linus Torvalds for his i386-based PC, and it was soon adopted as the kernel for the GNU o ...

in 2014, in kernel version 3.18. It was improved in version 4.0, bringing improvements necessary for e.g. the storage driver in Docker. While most

Live CD A live CD (also live DVD, live disc, or live operating system) is a complete bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than loading f ...

linux distributions used

Aufs aufs (short for advanced multi-layered unification filesystem) implements a union mount for Linux file systems. The name originally stood for AnotherUnionFS until version 2. Developed by Junjiro Okajima in 2006, aufs is a complete rewrite of t ...

as of November 2016,

Slackware Slackware is a Linux distribution created by Patrick Volkerding in 1993. Originally based on Softlanding Linux System, Slackware has been the basis for many other Linux distributions, most notably the first versions of SUSE Linux distributio ...

used overlayfs for its live CD.

Implementation

The main mechanics of OverlayFS relate to the merging of directory access when both filesystems present a directory for the same name. Otherwise, OverlayFS presents the object, if any, yielded by one or the other, with the "upper" filesystem taking precedence. Unlike some other overlay filesystems, the directory subtrees being merged by OverlayFS do not necessarily have to be from distinct filesystems. OverlayFS supports whiteouts and opaque directories in the upper filesystem to allow file and directory deletion. OverlayFS does not support renaming files without performing a full copy-up of the file; however, renaming directories in an upper filesystem has limited support. OverlayFS does not support merging changes from an upper filesystem to a lower filesystem.

References

External links

OverlayFS documentation
Free special-purpose file systems Linux kernel features Special-purpose file systems supported by the Linux kernel Union file systems Unix file system technology {{Linux-stub

History

Implementation

See also

References

External links