Docker and OverlayFS in practice

OverlayFS is a modern union filesystem that is similar to AUFS. In comparison to AUFS, OverlayFS:

• has a simpler design
• has been in the mainline Linux kernel since version 3.18
• is potentially faster

As a result, OverlayFS is rapidly gaining popularity in the Docker community and is seen by many as a natural successor to AUFS. As promising as OverlayFS is, it is still relatively young. Therefore caution should be taken before using it in production Docker environments.

Docker’s overlay storage driver leverages several OverlayFS features to build and manage the on-disk structures of images and containers.

Since version 1.12, Docker also provides overlay2 storage driver which is much more efficient than overlay in terms of inode utilization. The overlay2 driver is only compatible with Linux kernel 4.0 and later.

For comparison between overlay vs overlay2, please also refer to Select a storage driver.

Note: Since it was merged into the mainline kernel, the OverlayFS kernel module was renamed from “overlayfs” to “overlay”. As a result you may see the two terms used interchangeably in some documentation. However, this document uses “OverlayFS” to refer to the overall filesystem, and overlay/overlay2 to refer to Docker’s storage-drivers.

Image layering and sharing with OverlayFS (overlay)

OverlayFS takes two directories on a single Linux host, layers one on top of the other, and provides a single unified view. These directories are often referred to as layers and the technology used to layer them is known as a union mount. The OverlayFS terminology is “lowerdir” for the bottom layer and “upperdir” for the top layer. The unified view is exposed through its own directory called “merged”.

The diagram below shows how a Docker image and a Docker container are layered. The image layer is the “lowerdir” and the container layer is the “upperdir”. The unified view is exposed through a directory called “merged” which is effectively the containers mount point. The diagram shows how Docker constructs map to OverlayFS constructs.

Notice how the image layer and container layer can contain the same files. When this happens, the files in the container layer (“upperdir”) are dominant and obscure the existence of the same files in the image layer (“lowerdir”). The container mount (“merged”) presents the unified view.

The overlay driver only works with two layers. This means that multi-layered images cannot be implemented as multiple OverlayFS layers. Instead, each image layer is implemented as its own directory under /var/lib/docker/overlay. Hard links are then used as a space-efficient way to reference data shared with lower layers. As of Docker 1.10, image layer IDs no longer correspond to directory names in /var/lib/docker/.

To create a container, the overlay driver combines the directory representing the image’s top layer plus a new directory for the container. The image’s top layer is the “lowerdir” in the overlay and read-only. The new directory for the container is the “upperdir” and is writable.

Example: Image and container on-disk constructs (overlay)

The following docker pull command shows a Docker host with downloading a Docker image comprising five layers.

$ sudo docker pull ubuntu

Using default tag: latest
latest: Pulling from library/ubuntu

5ba4f30e5bea: Pull complete
9d7d19c9dc56: Pull complete
ac6ad7efd0f9: Pull complete
e7491a747824: Pull complete
a3ed95caeb02: Pull complete
Digest: sha256:46fb5d001b88ad904c5c732b086b596b92cfb4a4840a3abd0e35dbb6870585e4
Status: Downloaded newer image for ubuntu:latest

Each image layer has its own directory under /var/lib/docker/overlay/. This is where the contents of each image layer are stored.

The output of the command below shows the five directories that store the contents of each image layer just pulled. However, as can be seen, the image layer IDs do not match the directory names in /var/lib/docker/overlay. This is normal behavior in Docker 1.10 and later.

$ ls -l /var/lib/docker/overlay/

total 20
drwx------ 3 root root 4096 Jun 20 16:11 38f3ed2eac129654acef11c32670b534670c3a06e483fce313d72e3e0a15baa8
drwx------ 3 root root 4096 Jun 20 16:11 55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358
drwx------ 3 root root 4096 Jun 20 16:11 824c8a961a4f5e8fe4f4243dab57c5be798e7fd195f6d88ab06aea92ba931654
drwx------ 3 root root 4096 Jun 20 16:11 ad0fe55125ebf599da124da175174a4b8c1878afe6907bf7c78570341f308461
drwx------ 3 root root 4096 Jun 20 16:11 edab9b5e5bf73f2997524eebeac1de4cf9c8b904fa8ad3ec43b3504196aa3801

The image layer directories contain the files unique to that layer as well as hard links to the data that is shared with lower layers. This allows for efficient use of disk space.

$ ls -i /var/lib/docker/overlay/38f3ed2eac129654acef11c32670b534670c3a06e483fce313d72e3e0a15baa8/root/bin/ls

19793696 /var/lib/docker/overlay/38f3ed2eac129654acef11c32670b534670c3a06e483fce313d72e3e0a15baa8/root/bin/ls

$ ls -i /var/lib/docker/overlay/55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358/root/bin/ls

19793696 /var/lib/docker/overlay/55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358/root/bin/ls

Containers also exist on-disk in the Docker host’s filesystem under /var/lib/docker/overlay/. If you inspect the directory relating to a running container using the ls -l command, you find the following file and directories.

$ ls -l /var/lib/docker/overlay/<directory-of-running-container>

total 16
-rw-r--r-- 1 root root   64 Jun 20 16:39 lower-id
drwxr-xr-x 1 root root 4096 Jun 20 16:39 merged
drwxr-xr-x 4 root root 4096 Jun 20 16:39 upper
drwx------ 3 root root 4096 Jun 20 16:39 work

These four filesystem objects are all artifacts of OverlayFS. The “lower-id” file contains the ID of the top layer of the image the container is based on. This is used by OverlayFS as the “lowerdir”.

$ cat /var/lib/docker/overlay/ec444863a55a9f1ca2df72223d459c5d940a721b2288ff86a3f27be28b53be6c/lower-id

55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358

The “upper” directory is the containers read-write layer. Any changes made to the container are written to this directory.

The “merged” directory is effectively the containers mount point. This is where the unified view of the image (“lowerdir”) and container (“upperdir”) is exposed. Any changes written to the container are immediately reflected in this directory.

The “work” directory is required for OverlayFS to function. It is used for things such as copy_up operations.

You can verify all of these constructs from the output of the mount command. (Ellipses and line breaks are used in the output below to enhance readability.)

$ mount | grep overlay

overlay on /var/lib/docker/overlay/ec444863a55a.../merged
type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay/55f1e14c361b.../root,
upperdir=/var/lib/docker/overlay/ec444863a55a.../upper,
workdir=/var/lib/docker/overlay/ec444863a55a.../work)

The output reflects that the overlay is mounted as read-write (“rw”).

Image layering and sharing with OverlayFS (overlay2)

While the overlay driver only works with a single lower OverlayFS layer and hence requires hard links for implementation of multi-layered images, the overlay2 driver natively supports multiple lower OverlayFS layers (up to 128).

Hence the overlay2 driver offers better performance for layer-related docker commands (e.g. docker build and docker commit), and consumes fewer inodes than the overlay driver.

Example: Image and container on-disk constructs (overlay2)

After downloading a five-layer image using docker pull ubuntu, you can see six directories under /var/lib/docker/overlay2.

$ ls -l /var/lib/docker/overlay2

total 24
drwx------ 5 root root 4096 Jun 20 07:36 223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7
drwx------ 3 root root 4096 Jun 20 07:36 3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b
drwx------ 5 root root 4096 Jun 20 07:36 4e9fa83caff3e8f4cc83693fa407a4a9fac9573deaf481506c102d484dd1e6a1
drwx------ 5 root root 4096 Jun 20 07:36 e8876a226237217ec61c4baf238a32992291d059fdac95ed6303bdff3f59cff5
drwx------ 5 root root 4096 Jun 20 07:36 eca1e4e1694283e001f200a667bb3cb40853cf2d1b12c29feda7422fed78afed
drwx------ 2 root root 4096 Jun 20 07:36 l

The “l” directory contains shortened layer identifiers as symbolic links. These shortened identifiers are used for avoid hitting the page size limitation on mount arguments.

$ ls -l /var/lib/docker/overlay2/l

total 20
lrwxrwxrwx 1 root root 72 Jun 20 07:36 6Y5IM2XC7TSNIJZZFLJCS6I4I4 -> ../3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/diff
lrwxrwxrwx 1 root root 72 Jun 20 07:36 B3WWEFKBG3PLLV737KZFIASSW7 -> ../4e9fa83caff3e8f4cc83693fa407a4a9fac9573deaf481506c102d484dd1e6a1/diff
lrwxrwxrwx 1 root root 72 Jun 20 07:36 JEYMODZYFCZFYSDABYXD5MF6YO -> ../eca1e4e1694283e001f200a667bb3cb40853cf2d1b12c29feda7422fed78afed/diff
lrwxrwxrwx 1 root root 72 Jun 20 07:36 NFYKDW6APBCCUCTOUSYDH4DXAT -> ../223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7/diff
lrwxrwxrwx 1 root root 72 Jun 20 07:36 UL2MW33MSE3Q5VYIKBRN4ZAGQP -> ../e8876a226237217ec61c4baf238a32992291d059fdac95ed6303bdff3f59cff5/diff

The lowerest layer contains the “link” file which contains the name of the shortened identifier, and the “diff” directory which contains the contents.

$ ls /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/

diff  link

$ cat /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/link

6Y5IM2XC7TSNIJZZFLJCS6I4I4

$ ls  /var/lib/docker/overlay2/3a36935c9df35472229c57f4a27105a136f5e4dbef0f87905b2e506e494e348b/diff

bin  boot  dev  etc  home  lib  lib64  media  mnt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

The second layer contains the “lower” file for denoting the layer composition, and the “diff” directory for the layer contents. It also contains the “merged” and the “work” directories.

$ ls /var/lib/docker/overlay2/223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7

diff  link  lower  merged  work

$ cat /var/lib/docker/overlay2/223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7/lower

l/6Y5IM2XC7TSNIJZZFLJCS6I4I4

$ ls /var/lib/docker/overlay2/223c2864175491657d238e2664251df13b63adb8d050924fd1bfcdb278b866f7/diff/

etc  sbin  usr  var

A directory for running container have similar files and directories as well. Note that the lower list is separated by ‘:’, and ordered from highest layer to lower.

$ ls -l /var/lib/docker/overlay/<directory-of-running-container>

$ cat /var/lib/docker/overlay/<directory-of-running-container>/lower

l/DJA75GUWHWG7EWICFYX54FIOVT:l/B3WWEFKBG3PLLV737KZFIASSW7:l/JEYMODZYFCZFYSDABYXD5MF6YO:l/UL2MW33MSE3Q5VYIKBRN4ZAGQP:l/NFYKDW6APBCCUCTOUSYDH4DXAT:l/6Y5IM2XC7TSNIJZZFLJCS6I4I4

The result of mount is as follows:

$ mount | grep overlay

overlay on /var/lib/docker/overlay2/9186877cdf386d0a3b016149cf30c208f326dca307529e646afce5b3f83f5304/merged
type overlay (rw,relatime,
lowerdir=l/DJA75GUWHWG7EWICFYX54FIOVT:l/B3WWEFKBG3PLLV737KZFIASSW7:l/JEYMODZYFCZFYSDABYXD5MF6YO:l/UL2MW33MSE3Q5VYIKBRN4ZAGQP:l/NFYKDW6APBCCUCTOUSYDH4DXAT:l/6Y5IM2XC7TSNIJZZFLJCS6I4I4,
upperdir=9186877cdf386d0a3b016149cf30c208f326dca307529e646afce5b3f83f5304/diff,
workdir=9186877cdf386d0a3b016149cf30c208f326dca307529e646afce5b3f83f5304/work)

Container reads and writes with overlay

Consider three scenarios where a container opens a file for read access with overlay.

• The file does not exist in the container layer. If a container opens a file for read access and the file does not already exist in the container (“upperdir”) it is read from the image (“lowerdir”). This should incur very little performance overhead.

• The file only exists in the container layer. If a container opens a file for read access and the file exists in the container (“upperdir”) and not in the image (“lowerdir”), it is read directly from the container.

• The file exists in the container layer and the image layer. If a container opens a file for read access and the file exists in the image layer and the container layer, the file’s version in the container layer is read. This is because files in the container layer (“upperdir”) obscure files with the same name in the image layer (“lowerdir”).

Consider some scenarios where files in a container are modified.

• Writing to a file for the first time. The first time a container writes to an existing file, that file does not exist in the container (“upperdir”). The overlay/overlay2 driver performs a copy_up operation to copy the file from the image (“lowerdir”) to the container (“upperdir”). The container then writes the changes to the new copy of the file in the container layer.

  However, OverlayFS works at the file level not the block level. This means that all OverlayFS copy-up operations copy entire files, even if the file is very large and only a small part of it is being modified. This can have a noticeable impact on container write performance. However, two things are worth noting:

  • The copy_up operation only occurs the first time any given file is written to. Subsequent writes to the same file will operate against the copy of the file already copied up to the container.

  • OverlayFS only works with two layers. This means that performance should be better than AUFS which can suffer noticeable latencies when searching for files in images with many layers.

• Deleting files and directories. When files are deleted within a container a whiteout file is created in the containers “upperdir”. The version of the file in the image layer (“lowerdir”) is not deleted. However, the whiteout file in the container obscures it.

  Deleting a directory in a container results in opaque directory being created in the “upperdir”. This has the same effect as a whiteout file and effectively masks the existence of the directory in the image’s “lowerdir”.

• Renaming directories. Calling rename(2) for a directory is allowed only when both of the source and the destination path are on the top layer. Otherwise, it returns EXDEV (“cross-device link not permitted”).

So your application has to be designed so that it can handle EXDEV and fall back to a “copy and unlink” strategy.

Configure Docker with the overlay/overlay2 storage driver

To configure Docker to use the overlay storage driver your Docker host must be running version 3.18 of the Linux kernel (preferably newer) with the overlay kernel module loaded. For the overlay2 driver, the version of your kernel must be 4.0 or newer. OverlayFS can operate on top of most supported Linux filesystems. However, ext4 is currently recommended for use in production environments.

The following procedure shows you how to configure your Docker host to use OverlayFS. The procedure assumes that the Docker daemon is in a stopped state.

Caution: If you have already run the Docker daemon on your Docker host and have images you want to keep, push them to Docker Hub or your private Docker Trusted Registry before attempting this procedure.

1. If it is running, stop the Docker daemon.

2. Verify your kernel version and that the overlay kernel module is loaded.

    $ uname -r
   
    3.19.0-21-generic
   
    $ lsmod | grep overlay
   
    overlay
   

3. Start the Docker daemon with the overlay/overlay2 storage driver.

    $ dockerd --storage-driver=overlay &
   
    [1] 29403
    root@ip-10-0-0-174:/home/ubuntu# INFO[0000] Listening for HTTP on unix (/var/run/docker.sock)
    INFO[0000] Option DefaultDriver: bridge
    INFO[0000] Option DefaultNetwork: bridge
    <output truncated>
   

   Alternatively, you can force the Docker daemon to automatically start with the overlay/overlay2 driver by editing the Docker config file and adding the --storage-driver=overlay flag to the DOCKER_OPTS line. Once this option is set you can start the daemon using normal startup scripts without having to manually pass in the --storage-driver flag.

4. Verify that the daemon is using the overlay/overlay2 storage driver

    $ docker info
   
    Containers: 0
    Images: 0
    Storage Driver: overlay
     Backing Filesystem: extfs
    <output truncated>
   

   Notice that the Backing filesystem in the output above is showing as extfs. Multiple backing filesystems are supported but extfs (ext4) is recommended for production use cases.

Your Docker host is now using the overlay/overlay2 storage driver. If you run the mount command, you’ll find Docker has automatically created the overlay mount with the required “lowerdir”, “upperdir”, “merged” and “workdir” constructs.

OverlayFS and Docker Performance

As a general rule, the overlay/overlay2 drivers should be fast. Almost certainly faster than aufs and devicemapper. In certain circumstances it may also be faster than btrfs. That said, there are a few things to be aware of relative to the performance of Docker using the overlay/overlay2 storage drivers.

• Page Caching. OverlayFS supports page cache sharing. This means multiple containers accessing the same file can share a single page cache entry (or entries). This makes the overlay/overlay2 drivers efficient with memory and a good option for PaaS and other high density use cases.

• copy_up. As with AUFS, OverlayFS has to perform copy-up operations any time a container writes to a file for the first time. This can insert latency into the write operation — especially if the file being copied up is large. However, once the file has been copied up, all subsequent writes to that file occur without the need for further copy-up operations.

  The OverlayFS copy_up operation should be faster than the same operation with AUFS. This is because AUFS supports more layers than OverlayFS and it is possible to incur far larger latencies if searching through many AUFS layers.

• Inode limits. Use of the overlay storage driver can cause excessive inode consumption. This is especially so as the number of images and containers on the Docker host grows. A Docker host with a large number of images and lots of started and stopped containers can quickly run out of inodes. The overlay2 does not have such an issue.

Unfortunately you can only specify the number of inodes in a filesystem at the time of creation. For this reason, you may wish to consider putting /var/lib/docker on a separate device with its own filesystem, or manually specifying the number of inodes when creating the filesystem.

The following generic performance best practices also apply to OverlayFS.

• Solid State Devices (SSD). For best performance it is always a good idea to use fast storage media such as solid state devices (SSD).

• Use Data Volumes. Data volumes provide the best and most predictable performance. This is because they bypass the storage driver and do not incur any of the potential overheads introduced by thin provisioning and copy-on-write. For this reason, you should place heavy write workloads on data volumes.

OverlayFS compatibility

To summarize the OverlayFS’s aspect which is incompatible with other filesystems:

• open(2). OverlayFS only implements a subset of the POSIX standards. This can result in certain OverlayFS operations breaking POSIX standards. One such operation is the copy-up operation. Suppose that your application calls fd1=open("foo", O_RDONLY) and then fd2=open("foo", O_RDWR). In this case, your application expects fd1 and fd2 to refer to the same file. However, due to a copy-up operation that occurs after the first calling to open(2), the descriptors refer to different files.

yum is known to be affected unless the yum-plugin-ovl package is installed. If the yum-plugin-ovl package is not available in your distribution (e.g. RHEL/CentOS prior to 6.8 or 7.2), you may need to run touch /var/lib/rpm/* before running yum install.

• rename(2). OverlayFS does not fully support the rename(2) system call. Your application needs to detect its failure and fall back to a “copy and unlink” strategy.