OverlayFS storage driver
OverlayFS is a union filesystem.
This page refers to the Linux kernel driver as OverlayFS
and to the Docker
storage driver as overlay2
.
Note
For
fuse-overlayfs
driver, check Rootless mode documentation.
Prerequisites
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 onxfs
backing filesystems, but only withd_type=true
enabled.Use
xfs_info
to verify that theftype
option is set to1
. 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.
Configure Docker with the overlay2
storage driver
Before following this procedure, you must first meet all the prerequisites.
The following steps outline how to configure the overlay2
storage driver.
Stop Docker.
$ sudo systemctl stop docker
Copy the contents of
/var/lib/docker
to a temporary location.$ cp -au /var/lib/docker /var/lib/docker.bk
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.Edit
/etc/docker/daemon.json
. If it doesn't yet exist, create it. Assuming that the file was empty, add the following contents.{ "storage-driver": "overlay2" }
Docker doesn't start if the
daemon.json
file contains invalid JSON.Start Docker.
$ sudo systemctl start docker
Verify that the daemon is using the
overlay2
storage driver. Use thedocker info
command and look forStorage Driver
andBacking filesystem
.$ docker info Containers: 0 Images: 0 Storage Driver: overlay2 Backing Filesystem: xfs Supports d_type: true Native Overlay Diff: true <...>
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.
How the overlay2
driver works
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 a 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
.
Warning
Don't directly manipulate any files or directories within
/var/lib/docker/
. These files and directories are managed by Docker.
$ 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 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.
$ 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 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.
$ 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-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.
$ 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
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.
$ 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)
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 the
upperdir
. 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.
$ 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
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.
Warning
Don't directly manipulate any files or directories within
/var/lib/docker/
. These files and directories are managed by Docker.
$ 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 shared with lower layers. This allows for efficient use of disk space.
$ ls -i /var/lib/docker/overlay2/38f3ed2eac129654acef11c32670b534670c3a06e483fce313d72e3e0a15baa8/root/bin/ls
19793696 /var/lib/docker/overlay2/38f3ed2eac129654acef11c32670b534670c3a06e483fce313d72e3e0a15baa8/root/bin/ls
$ ls -i /var/lib/docker/overlay2/55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358/root/bin/ls
19793696 /var/lib/docker/overlay2/55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358/root/bin/ls
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:
$ ls -l /var/lib/docker/overlay2/<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
The lower-id
file contains the ID of the top layer of the image the container
is based on, which is the OverlayFS lowerdir
.
$ cat /var/lib/docker/overlay2/ec444863a55a9f1ca2df72223d459c5d940a721b2288ff86a3f27be28b53be6c/lower-id
55f1e14c361b90570df46371b20ce6d480c434981cbda5fd68c6ff61aa0a5358
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.
$ mount | grep overlay
overlay on /var/lib/docker/overlay2/l/ec444863a55a.../merged
type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay2/l/55f1e14c361b.../root,
upperdir=/var/lib/docker/overlay2/l/ec444863a55a.../upper,
workdir=/var/lib/docker/overlay2/l/ec444863a55a.../work)
The rw
on the second line shows that the overlay
mount is read-write.
How container reads and writes work with overlay2
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 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 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 thelowerdir
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.
OverlayFS and Docker Performance
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 overlay2
drivers 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.
Limitations on 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 thenfd2=open("foo", O_RDWR)
. In this case, your application expectsfd1
andfd2
to refer to the same file. However, due to a copy-up operation that occurs after the second calling toopen(2)
, the descriptors refer to different files. Thefd1
continues to reference the file in the image (lowerdir
) and thefd2
references the file in the container (upperdir
). A workaround for this is totouch
the files which causes the copy-up operation to happen. All subsequentopen(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 theyum-plugin-ovl
package is installed. If theyum-plugin-ovl
package is not available in your distribution such as RHEL/CentOS prior to 6.8 or 7.2, you may need to runtouch /var/lib/rpm/*
before runningyum install
. This package implements thetouch
workaround referenced above foryum
. 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.