Mon 08 December 2014 | -- (permalink)

These are some notes to help myself remember the basic usage and mechanics of overlayfs, as well as explain them to others.

These steps are performed in an Ubuntu 14.04 (Trusty) Vagrant box, which comes with OverlayFS support out of the box.

First make 3 folders:

```

mkdir base_folder

mkdir overlay_folder

mkdir work_folder

```

Put some dummy files in the base and overlay folders. We'll watch how their contents change after later steps.

```

echo "I am in the base" > base_folder/base_file

echo "I am in the overlay" > overlay_folder/overlay_file

```

Now mount the overlayfs

```

mount -t overlayfs -o lowerdir=/home/vagrant/base_folder,upperdir=/home/vagrant/overlay_folder overlayfs /home/vagrant/work_folder

```

Now in the work folder we should see files from both the base and the overlay:

```

ls work_folder/

base_file overlay_file ```

Now let's try making a new file in the work folder mount point.

```

echo "i am in the work folder" > work_folder/work_file

```

You should see the new file in the work folder, as well as in the overlay folder.

```

ls work_folder/

base_file overlay_file work_file

ls overlay_folder/

overlay_file work_file ```

You will not see the new file in the base.

```

ls base_folder/

base_file ```

Now let's try modifying a file that is in the base, from the work folder mountpoint.

```

echo "this is new content in the base file" > work_folder/base_file

```

You'll see your changes in the work folder and the overlay folder, but not in the base.

```

cat work_folder/base_file

this is new content in the base file

cat overlay_folder/base_file

this is new content in the base file

cat base_folder/base_file

I am in the base ```

Now try modifying a file from the overlay, while in the work folder.

```

echo "this is new content in the overlay file" > work_folder/overlay_file

```

You'll see that your changes exist in the work folder and in the overlay, but the file still doesn't exist in the base.

```

cat work_folder/overlay_file

this is new content in the overlay file

cat overlay_folder/overlay_file

this is new content in the overlay file

ls base_folder/

base_file ```

OverlayFS is used by the Velociraptor project that we've built at YouGov, which provides a Heroku-compatible platform for deploying applications on Linux.

OverlayFS is expected to be part of Linux kernel 3.18, though you can get it now in Ubuntu.