|Developer(s)||Miklos Szeredi, Open Source Community|
|Full name||Overlay File System|
|File system permissions||POSIX|
|Supported operating systems||Linux, FreeBSD and NetBSD|
In computing, OverlayFS is a union mount filesystem implementation for Linux. 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.
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 had already adopted it for their use.
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 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.
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