UnionFS
| Developer(s) | Open Source Community |
|---|---|
| Full name | Unification File System |
| Features | |
| File system permissions | POSIX |
| Transparent compression | No |
| Transparent encryption | No (but can be provided at the block device level) |
| Other | |
| Supported operating systems | Linux, FreeBSD and NetBSD |
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.
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 (knoppix-data.img for Knoppix 7) on a writable drive (such as a USB memory stick) 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 (compared to full VMs, for example).
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