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Linux AuFS Examples: Another Union File System Tutorial (UnionFS Implementation)

AuFS stands for Another Union File System.

AuFS started as an implementation of UnionFS Union File System.

An union filesystem takes an existing filesystem and transparently overlays it on a newer filesystem. It allows files and directories of separate filesystem to co-exist under a single roof. AuFS can merge several directories and provide a single merged view of it.

AuFS is used in many of the opensource projects like, Slax, Knoppix, and many other live CD and live USB distributions.

On Debian based systems, for example on Ubuntu, do the following to install aufs.

# apt-get install aufs-tools

Example 1 – Understanding How AuFS Works

This example shows how to mount two directories of a same filesystem.

# mkdir /tmp/dir1

# mkdir /tmp/aufs-root

# mount -t aufs -o br=/tmp/dir1:/home/lakshmanan none /tmp/aufs-root/

The first two commands created 2 new directories. The mount.aufs is the command to mount the filesystem as Union mount.

The mount command, specifies it is going to union mount “/tmp/dir1″ and /home/lakshmanan” under “/tmp/aufs-root”. The directory “/tmp/aufs-root” will have the content of both “/tmp/dir1” and “/home/lakshmanan”.

The following options are used in the above mount command example:

  • -o – specifies options to be passed to the filesystem
  • br – specifies a branch, where each branch is separated by colon (:). A branch is nothing but a directory on a system.
  • none – specifies we don’t have any device associated with it, since we are going to mount two directories

As you see from the below ls -l command output, we can see that aufs is merging the contents of the 2 separate directories and brings a unified view.

# ls -l /tmp/dir1/
-rw-r--r-- 1 root       root       23 Mar 25 14:21 file_in_tmp_dir1

# ls -l /home/lakshmanan
-rw-r--r-- 1 root       root            26 Mar 25 14:20 file_in_home_dir

# ls -l /tmp/aufs-root/
-rw-r--r-- 1 root       root            26 Mar 25 14:20 file_in_home_dir
-rw-r--r-- 1 root       root            23 Mar 25 14:21 file_in_tmp_dir1

By default, if no permissions are specified, first branch will be mounted as writable, and the remaining branches will be mounted as readonly.

So when you create any file inside ‘/tmp/aufs-root/’, physically it will be created under “/tmp/dir1”, since it is the only writable branch.

Example 2 – Unified View of Home Directories

At times, system administrators end up having multiple disk partitions which has home directories of multiple users. We will see an example of how we can make it as unified view and simplify the admin process.

In this example:

  • /home -> is the mount point of /dev/sda2 having “lakshmanan” and “sysadmin” users
  • /home1 -> is the mount point of /dev/sdb2 having “test” user.
# mount -t aufs -o br=/home=rw:/home1=rw -o udba=reval  none /common/

# ls -l /common/
drwxr-xr-x 39 lakshmanan lakshmanan  4096 Mar 25 15:52 lakshmanan
drwxr-xr-x 26 sysadmin   sysadmin    4096 Mar 25 15:51 sysadmin
drwxr-xr-x  2 root       root        4096 Mar 25 16:36 test

The above mount command has an extra option called “udba”, which refers to “User’s Direct Branch Access”. This option says what to do, if the user directly access a branch and create/update files without going through AuFS.

The following are possible values for udba:

  • udba=none – With this option, the AuFS will be faster, but may show incorrect data, if the user created any files/directories without going through the AuFS.
  • udba=reval – With this option, the AuFS will re-lookup the branches and update it. So any changes done on any directories within the branch, will be reflected in “/common”.
  • udba=notify – With this option, the AuFS will register for inotify for all the directories in the branches. This affects the performance of AuFS.
# touch /home/lakshmanan/TGS

# ls -l /common/lakshmanan/
-rw-r--r-- 1 root       root             0 Mar 25 17:17 TGS

The touch command created a file named “TGS” in the home directory of “lakshmanan” without going through AuFS. Since we mounted using “udba=retval”, when we execute ls, AuFS did a re-lookup and displays the new file that was created.

Also note that, in the previous mount command, we specified the permission for each branches as readwrite. So when a file is created under /common/test/, it will be physically created in “/home1/test/”, which is the actual physical location. Same is applicable for other directories also.

# touch /common/test/Testing

# ls -l /home1/test/
-rw-r--r-- 1 root root  0 Mar 25 18:26 Testing

Example 3 – Mount with readonly Permission for Branches

You can also mount by having permissions set for each branches.

# mount -t aufs -o br=/tmp/dir1=rw:/home/lakshmanan=ro -o udba=reval none /tmp/aufs-root/

The above command will mount the /tmp/aufs-root, by having “/tmp/dir1” as writable, and “/home/lakshmanan” as readonly. Any change you do in /tmp/aufs-root, will be saved only under “/tmp/dir1”, since that is the only writable directory.

The following sequence of commands can be used to verify the above.

# cat > /home/lakshmanan/1.txt
This is a new file

# cat /tmp/aufs-root/1.txt
This is a new file

# cat >> /tmp/aufs-root/1.txt
This is updated on AuFS space

# cat /home/lakshmanan/1.txt
This is a new file

# cat /tmp/dir1/1.txt
This is a new file
This is updated on AuFS space

In the above sequence of commands, we did the following:

  • We are creating a file called 1.txt under “/home/lakshmanan”.
  • This file is reflected in the union mount directory, because of the udba=retval option that we explained above
  • Update the file that is present in the union mount directory
  • Even tough the file is present inside /home/lakshmanan, since it is mounted as readonly, AuFS takes a copy of the file and places it on “/tmp/dir1”.
  • On top of that copy, it appends the content, and saves the file
  • In /home/lakshmanan/1.txt, the change is not reflected
  • A new file named “1.txt”, is created under “/tmp/dir1” which has the updated content

Example 4 – Apply Round Robin Policy for Creating files under AuFS

When we have more than 2 branches which are writable, we can choose any one of the predefined policy, so that a file created will be stored based on the policy chosen.

# mount -t aufs -o br=/tmp/dir1=rw:/home/lakshmanan=rw -o udba=reval -o create=rr none /tmp/aufs-root/

The option “create=rr”, specifies that round robin policy has to be applied for this union mount. In round robin policy, if we create 4 files, 2 files will be in “/tmp/dir1” and 2 files will be in “/home/lakshmanan”.

# touch /tmp/aufs/first-round-robin

# touch /tmp/aufs/second-round-robin

# ls -l /tmp/dir1/first-round-robin
-rw-r--r-- 1 root root 0 Mar 25 21:53 /tmp/dir1/first-round-robin

# ls -l /home/lakshmanan/second-round-robin
-rw-r--r-- 1 root root 0 Mar 25 21:54 /home/lakshmanan/second-round-robin
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Comments on this entry are closed.

  • Benny Helms May 8, 2013, 9:10 am

    I’ve been a sysadmin for decades and this is the first time I’ve heard of this! Thank you for sharing this information!

  • Jalal Hajigholamali May 8, 2013, 9:23 am

    Thanks a lot,

    I could not find this beautiful(aufs-tools) package under Redhat

  • bob May 8, 2013, 10:44 am

    Great article. Thanks!!!

  • Timo May 8, 2013, 4:06 pm

    Fantastic article as always!

    But one point bothers me regarding your “Example 1”.
    What does aufs do, if in both directories are files with the same name?
    I.e. you have in both directories “/tmp/dir1/” and “/home/lakshmanan” a file called “test.file” and you mount this with
    # mount -t aufs -o br=/tmp/dir1:/home/lakshmanan none /tmp/aufs-root/

  • Lakshmanan Ganapathy May 8, 2013, 11:09 pm


    It will take the test.file from /tmp/dir1, since you have given that as the first branch when you mount.

    If you interchange the order, mount -t aufs -o br=/home/lakshmanan:/tmp/dir1 none /tmp/aufs-root/, then it will take from /home/lakshmanan.

  • dru8274 May 13, 2013, 5:01 am

    Aufs is a lot of fun. But just to mention that programs which create hard-links can sometimes lead to corrupted files in aufs. This can happen when the two hard-linked files are on different branches/layers in aufs, which outside of aufs are located on separate partitions.

  • cgraff May 22, 2013, 1:05 pm

    Thank you so much for this. Is it possible to cajole you into giving some simple example for using aufs from /etc/fstab ? I specifically would like to mount a writable branch for a filesystem.squashfs (persistencey). Here is my mount so far:
    cat > /etc/fstab <<EOF
    /filesystem.squashfs /mnt squashfs ro 0 0
    This mounts it at /mnt but i am not really good enough with /etc/fstab syntax to extrapolate your examples to it properly. Again thanks for this work, it has been a great help.

  • cgraff May 22, 2013, 9:10 pm

    Got it, this seems to do what i want:

    cat > /etc/fstab <<EOF
    /filesystem.squashfs /mnt squashfs ro 0 0
    none /media aufs dirs=/media:/mnt 0 0

    But if you have any ideas on how to that with "/" from the debian intramfs, or suggestions, it would be much appreciated 🙂 . Again thank you for your article, I would not have gotten this working so quickly without your clear examples.

  • Lakshmanan Ganapathy May 22, 2013, 11:10 pm


    Can you please elaborate on what you need to do?. I didn’t get what you said. Are you asking how to mount from initramfs??

  • cgraff May 23, 2013, 9:03 am

    my hypothesis:
    In order for the kernel to boot a /filesystem.squashfs as its root “/” it needs to fisrt be mounted in the initramfs. For debian liveCD’s this is usually done with a program called `live-boot’ . The liveCD uses aufs to union mount the fileystem.squashfs, with a persistence volume. and if i am not mistaken, the users system is actually the filesystem.sqaushfs mounted to ram, and union mounted with aufs to the persistency layer.
    I am trying to write a model system for how to do this with my own aufs mounts without using this tool `live-boot’.

  • Steve May 23, 2013, 7:04 pm

    Thank you for this – very helpful

  • Graham July 28, 2013, 11:06 am

    I appreciate this posting. One thing perhaps worth including for us aufs newbies is that folders must be unmounted before deletion

    sudo umount /tmp/aufs-root/ && rm -rf /tmp/aufs-root/

  • Niels Böhm September 21, 2013, 6:09 am


    Please not that it’s dangerous to “rm -rf” an empty directory (that was a mountpoint). What if you forgot to unmount it first? You’ll delete all contents.

    There’s a better tool for that: rmdir. You should always use “rmdir” for removing empty directories, for if you’re mistaken and the directory is not actually empty, rmdir will just bail out with an error message without doing anything instead of deleting your data.

  • Kayot October 24, 2013, 9:06 pm

    I hate to revive an old thread but I’m kind of out of options. I posted to the Ubuntu forums, but I’ll repost here in case someone knows an answer.

    I’m migrating my DrivePool drives 5+1 (5 Storage, 1 SnapRaid) to an old system I’ve put Ubuntu Server on. I can mount multiple drives just fine (I’m still doing tests before the shift) using this script:


    mount -t aufs -o br=$d0=rw:$d1=rw:$d2=rw:$d3=rw:$d4=rw -o create=mfs none pool

    I put the pool directory into a Samba share. The problem is that, the reported free space is only listing the first drives free space. Is there a way to pool the free space so that it reports all the available space?

  • Paco November 25, 2013, 5:06 pm

    @Kayot, I think “-o sum” will do what you want

  • cheneydeng November 28, 2013, 4:26 am

    fantastic article,and i think here “/home1 -> is the mount point of /dev/sdb2 having “test” user.” should be “root” user.

  • Kayot December 1, 2013, 4:20 pm

    Ah, I was doing it wrong. I was trying to add sum to the line without using -o. My final line looked like:

    mount -t aufs -o br=$d0=rw:$d1=rw:$d2=rw:$d3=rw:$d4=rw -o create=mfs -o sum none pool

    I didn’t realize I needed a -o after every option. Such a noobish mistake. Now to hunt down my other post and report the way to solve it for future generations. Thank you.

  • Amit Naudiyal January 13, 2017, 12:05 am

    mount command with ‘aufs’ type doesn’t work on Ubuntu 16.04. It says:

    mount: unknown filesystem type ‘aufs’