OverlayFS storage driver (original) (raw)

OverlayFS is a union filesystem.

This page refers to the Linux kernel driver as OverlayFS and to the Docker storage driver as overlay2.

For fuse-overlayfs driver, checkRootless mode documentation.

OverlayFS is the recommended storage driver, and supported if you meet the following prerequisites:

• Version 4.0 or higher of the Linux kernel, or RHEL or CentOS using version 3.10.0-514 of the kernel or higher.
• The overlay2 driver is supported on xfs backing filesystems, but only with d_type=true enabled.
  Use xfs_info to verify that the ftype option is set to 1. To format anxfs filesystem correctly, use the flag -n ftype=1.
• Changing the storage driver makes existing containers and images inaccessible on the local system. Use docker save to save any images you have built or push them to Docker Hub or a private registry before changing the storage driver, so that you don't need to re-create them later.

Before following this procedure, you must first meet all theprerequisites.

The following steps outline how to configure the overlay2 storage driver.

1. Stop Docker.
2. Copy the contents of /var/lib/docker to a temporary location.
3. If you want to use a separate backing filesystem from the one used by/var/lib/, format the filesystem and mount it into /var/lib/docker. Make sure to add this mount to /etc/fstab to make it permanent.
4. Edit /etc/docker/daemon.json. If it doesn't yet exist, create it. Assuming that the file was empty, add the following contents.
   Docker doesn't start if the daemon.json file contains invalid JSON.
5. Start Docker.
6. Verify that the daemon is using the overlay2 storage driver. Use the docker info command and look for Storage Driver andBacking filesystem.

Docker is now using the overlay2 storage driver and has automatically created the overlay mount with the required lowerdir, upperdir, merged, and workdir constructs.

Continue reading for details about how OverlayFS works within your Docker containers, as well as performance advice and information about limitations of its compatibility with different backing filesystems.

OverlayFS layers two directories on a single Linux host and presents them as a single directory. These directories are called layers, and the unification process is referred to as a union mount. OverlayFS refers to the lower directory as lowerdir and the upper directory as upperdir. The unified view is exposed through its own directory called merged.

The overlay2 driver natively supports up to 128 lower OverlayFS layers. This capability provides better performance for layer-related Docker commands such as docker build and docker commit, and consumes fewer inodes on the backing filesystem.

Image and container layers on-disk

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

Don't directly manipulate any files or directories within/var/lib/docker/. These files and directories are managed by Docker.

The new l (lowercase L) directory contains shortened layer identifiers as symbolic links. These identifiers are used to avoid hitting the page size limitation on arguments to the mount command.

The lowest layer contains a file called link, which contains the name of the shortened identifier, and a directory called diff which contains the layer's contents.

The second-lowest layer, and each higher layer, contain a file called lower, which denotes its parent, and a directory called diff which contains its contents. It also contains a merged directory, which contains the unified contents of its parent layer and itself, and a work directory which is used internally by OverlayFS.

To view the mounts which exist when you use the overlay storage driver with Docker, use the mount command. The output below is truncated for readability.

The rw on the second line shows that the overlay mount is read-write.

The following diagram shows how a Docker image and a Docker container are layered. The image layer is the lowerdir and the container layer is theupperdir. If the image has multiple layers, multiple lowerdir directories are used. The unified view is exposed through a directory called merged which is effectively the containers mount point.

Where the image layer and the container layer contain the same files, the container layer (upperdir) takes precedence and obscures the existence of the same files in the image layer.

To create a container, the overlay2 driver combines the directory representing the image's top layer plus a new directory for the container. The image's layers are the lowerdirs in the overlay and are read-only. The new directory for the container is the upperdir and is writable.

Image and container layers on-disk

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

The image layers

Each image layer has its own directory within /var/lib/docker/overlay/, which contains its contents, as shown in the following example. The image layer IDs don't correspond to the directory IDs.

Don't directly manipulate any files or directories within/var/lib/docker/. These files and directories are managed by Docker.

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

The container layer

Containers also exist on-disk in the Docker host's filesystem under/var/lib/docker/overlay/. If you list a running container's subdirectory using the ls -l command, three directories and one file exist:

The lower-id file contains the ID of the top layer of the image the container is based on, which is the OverlayFS lowerdir.

The upper directory contains the contents of the container's read-write layer, which corresponds to the OverlayFS upperdir.

The merged directory is the union mount of the lowerdir and upperdirs, which comprises the view of the filesystem from within the running container.

The work directory is internal to OverlayFS.

To view the mounts which exist when you use the overlay2 storage driver with Docker, use the mount command. The following output is truncated for readability.

The rw on the second line shows that the overlay mount is read-write.

Reading files

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 incurs 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's read directly from the container.

The file exists in both 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. Files in the container layer (upperdir) obscure files with the same name in the image layer (lowerdir).

Modifying files or directories

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 overlay2 driver performs acopy_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 rather than the block level. This means that all OverlayFS copy_up operations copy the entire file, even if the file is large and only a small part of it's 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 a given file is written to. Subsequent writes to the same file operate against the copy of the file already copied up to the container.
• OverlayFS works with multiple layers. This means that performance can be impacted when searching for files in images with many layers.

Deleting files and directories

• When a file is deleted within a container, a whiteout file is created in the container (upperdir). The version of the file in the image layer (lowerdir) is not deleted (because the lowerdir is read-only). However, the whiteout file prevents it from being available to the container.
• When a directory is deleted within a container, an opaque directory is created within the container (upperdir). This works in the same way as a whiteout file and effectively prevents the directory from being accessed, even though it still exists in the image (lowerdir).

Renaming directories

Calling rename(2) for a directory is allowed only when both the source and the destination path are on the top layer. Otherwise, it returns EXDEV error ("cross-device link not permitted"). Your application needs to be designed to handle EXDEV and fall back to a "copy and unlink" strategy.

overlay2 may perform better than btrfs. However, be aware of the following details:

Page caching

OverlayFS supports page cache sharing. Multiple containers accessing the same file share a single page cache entry for that file. This makes the overlay2drivers efficient with memory and a good option for high-density use cases such as PaaS.

Copyup

As with other copy-on-write filesystems, OverlayFS performs copy-up operations whenever a container writes to a file for the first time. This can add latency into the write operation, especially for large files. However, once the file has been copied up, all subsequent writes to that file occur in the upper layer, without the need for further copy-up operations.

Performance best practices

The following generic performance best practices apply to OverlayFS.

Use fast storage

Solid-state drives (SSDs) provide faster reads and writes than spinning disks.

Use volumes for write-heavy workloads

Volumes provide the best and most predictable performance for write-heavy workloads. This is because they bypass the storage driver and don't incur any of the potential overheads introduced by thin provisioning and copy-on-write. Volumes have other benefits, such as allowing you to share data among containers and persisting your data even if no running container is using them.

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 callsfd1=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 second calling to open(2), the descriptors refer to different files. The fd1 continues to reference the file in the image (lowerdir) and the fd2 references the file in the container (upperdir). A workaround for this is to touch the files which causes the copy-up operation to happen. All subsequent open(2) operations regardless of read-only or read-write access mode reference the file in the container (upperdir).

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 such as RHEL/CentOS prior to 6.8 or 7.2, you may need to run touch /var/lib/rpm/*before running yum install. This package implements the touch workaround referenced above for yum.

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.