OverlayFS
Overlayfs (Overlay Filesystem) is an in-kernel attempt at providing union file system capabilities on Linux. OverlayFS differs from other union filesystem implementations in that after a file is opened all operations go directly to the underlying, lower or upper, filesystems. This simplifies the implementation and allows native performance in these cases.[1]
The option to enable OverlayFS exists in Linux kernels 3.18 and higher.[2]
Installation
Kernel
File systems --->
[*] Overlay filesystem support
Usage
Once enabled in the kernel OverlayFS can be controlled using the mount command.
root #mount -t overlay overlay -o lowerdir=lowerdir,upperdir=upperdir,workdir=workdir mountpointExample
To mount an overlay filesystem using the following example of a structure on an ext4 base filesystem.
Create the following folder structure:
user $tree test_foldertest_folder ├── low ├── my_overlay └── up
On the folder low, create a file with a clear and recognizable name. Repeat the step on the folder up to get a structure similar to the following:
user $tree test_foldertest_folder
├── low
│ └── low_file
├── my_overlay
└── up
└── up_fileHaving that tree, the following command will create an overlay structure with the up folder above the low folder and that structure will be on the my_overlay folder.
root #mount -t overlay overlay -o lowerdir=/test_folder/low,upperdir=/test_folder/up,workdir=/test_folder/my_overlay /test_folder/my_overlay/After inspecting the tree structure of the test_folder, this will be printed:
user $tree test_foldertest_folder
├── low
│ └── low_file
├── my_overlay
│ ├── low_file
│ └── up_file
└── up
└── up_file
A file can be created using the normal filesystem structure, like the following
root #touch my_overlay/my_overlay_fileand will generate the following tree
user $tree test_foldertest_folder
├── low
│ └── low_file
├── my_overlay
│ ├── low_file
│ ├── my_overlay_file
│ └── up_file
└── up
├── my_overlay_file
└── up_file
The overlay working dir can be unmounted with the umount command
root #umount /test_folder/my_overlay/After unmounting the overlay folder, a new subfolder will appear on the directory where the operation was conducted
user $tree test_folder|
test_folder
├── low
│ └── low_file
├── my_overlay
│ └── work
└── up
├── my_overlay_file
└── up_file
That folder will have the following properties
d--------- 2 root root 4,0K sep 6 09:54 work
Multiple lowerdir entries, colon (:) separated, can be used. When doing so, upperdir and workdir can be omitted to make a read-only mount.
When included, both upperdir and workdir have to reside within the same filesystem.
See also
- Aufs — an advanced multi-layered unification filesystem.
- SquashFS — an open source, read only, extremely compressible filesystem.
- Wikipedia:UnionFS — The original union filesystem.
External resources
- A LWN article written by Jonathan Corbet in June 2011 covering vises and virtues of OverlayFS
- An informative AskUbuntu.com thread
- Overlay fs in the Linux git repository