UnionFS
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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... Wikipedia
Developer(s): Open Source Community
File system permissions: POSIX
Supported operating systems: Linux, FreeBSD and NetBSD
Transparent encryption: No (but can be provided at the block device level)