Linux AUFS

1. History of Linux AUFS

Because UnionFS had poor reliability and performance, AUFS is effectively a rewrite of UnionFS.

2. What Is Linux AUFS?

Advanced multi-layered unification filesystem is an implementation of union mount for the Linux filesystem. A union mount is a method of combining multiple directories into one directory. Externally, this directory appears to contain the contents of all the directories.

3. Why Linux AUFS Is Needed

4. How to Use Linux AUFS

1. Create directories and files

   mkdir aufs
   mkdir aufs/mnt
   mkdir aufs/container-layer
   echo "I am container layer" > aufs/container-layer/container-layer.txt
   mkdir aufs/{image-layer1,image-layer2,image-layer3}
   echo "I am image layer 1" > aufs/image-layer1/image-layer1.txt
   echo "I am image layer 2" > aufs/image-layer2/image-layer2.txt
   echo "I am image layer 3" > aufs/image-layer3/image-layer3.txt

2. Mount as AUFS

   cd aufs
   sudo mount -t aufs -o dirs=./container-layer:./image-layer1:./image-layer2:./image-layer3 none ./mnt

3. View the contents of the mounted directory

   tree mnt

4. Modify the contents of a file under mnt

   echo "I changed mnt/image-layer2.txt" >> mnt/image-layer2.txt

5. View the file contents under mnt and under image-layer2

   # changed
   cat mnt/image-layer2.txt
   # unchanged
   cat image-layer2/image-layer2.txt

6. View the container-layer directory

   tree container-layer

5. Linux AUFS Principle

5.1. Copy-on-Write

If a resource is duplicated, there is no need to immediately copy a new resource instance before the resource is modified. The resource is shared by different owners. When any owner wants to modify the resource, a new resource instance is copied for that owner to modify. The modified resource becomes that owner’s private resource. By sharing resources in this way, the cost of copying identical resources can be significantly reduced, but doing so also adds some overhead when the resource is modified.

6. References

• aufs - Wikipedia
• Union mount - Wikipedia