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Add overlay2 description to overlayfs-driver.md 

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>
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Akihiro Suda 2016-06-20 17:29:21 +09:00
parent 2cb050cde3
commit 8625adbd67
1 changed files with 146 additions and 57 deletions
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docs/userguide/storagedriver/overlayfs-driver.md
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@ -25,13 +25,20 @@ 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](selectadriver.md#overlay-vs-overlay2).
>**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` to refer
> to Docker's storage-driver.
> uses "OverlayFS" to refer to the overall filesystem, and `overlay`/`overlay2`
> to refer to Docker's storage-drivers.
## Image layering and sharing with OverlayFS
## 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
@ -53,52 +60,60 @@ Notice how the image layer and container layer can contain the same files. When
obscure the existence of the same files in the image layer ("lowerdir"). The
container mount ("merged") presents the unified view.
OverlayFS 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/`
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
### Example: Image and container on-disk constructs (`overlay`)
The following `docker pull` command shows a Docker host with downloading a
Docker image comprising four layers.
Docker image comprising five layers.
$ sudo docker pull ubuntu
Using default tag: latest
latest: Pulling from library/ubuntu
8387d9ff0016: Pull complete
3b52deaaf0ed: Pull complete
4bd501fad6de: Pull complete
5ba4f30e5bea: Pull complete
9d7d19c9dc56: Pull complete
ac6ad7efd0f9: Pull complete
e7491a747824: Pull complete
a3ed95caeb02: Pull complete
Digest: sha256:457b05828bdb5dcc044d93d042863fba3f2158ae249a6db5ae3934307c757c54
Digest: sha256:46fb5d001b88ad904c5c732b086b596b92cfb4a4840a3abd0e35dbb6870585e4
Status: Downloaded newer image for ubuntu:latest
Each image layer has it's 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 four directories that store the
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 24
drwx------ 3 root root 4096 Oct 28 11:02 1d073211c498fd5022699b46a936b4e4bdacb04f637ad64d3475f558783f5c3e
drwx------ 3 root root 4096 Oct 28 11:02 5a4526e952f0aa24f3fcc1b6971f7744eb5465d572a48d47c492cb6bbf9cbcda
drwx------ 5 root root 4096 Oct 28 11:06 99fcaefe76ef1aa4077b90a413af57fd17d19dce4e50d7964a273aae67055235
drwx------ 3 root root 4096 Oct 28 11:01 c63fb41c2213f511f12f294dd729b9903a64d88f098c20d2350905ac1fdbcbba
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
@ -106,17 +121,17 @@ directories.
$ ls -l /var/lib/docker/overlay/<directory-of-running-container>
total 16
-rw-r--r-- 1 root root 64 Oct 28 11:06 lower-id
drwxr-xr-x 1 root root 4096 Oct 28 11:06 merged
drwxr-xr-x 4 root root 4096 Oct 28 11:06 upper
drwx------ 3 root root 4096 Oct 28 11:06 work
-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/73de7176c223a6c82fd46c48c5f152f2c8a7e49ecb795a7197c3bb795c4d879e/lower-id
1d073211c498fd5022699b46a936b4e4bdacb04f637ad64d3475f558783f5c3e
$ 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.
@ -133,13 +148,85 @@ 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/73de7176c223.../merged
type overlay (rw,relatime,lowerdir=/var/lib/docker/overlay/1d073211c498.../root,
upperdir=/var/lib/docker/overlay/73de7176c223.../upper,
workdir=/var/lib/docker/overlay/73de7176c223.../work)
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.
$ /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
@ -164,9 +251,9 @@ 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` 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.
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
@ -191,13 +278,13 @@ file in the image layer ("lowerdir") is not deleted. However, the whiteout file
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".
## Configure Docker with the overlay storage driver
## Configure Docker with the `overlay`/`overlay2` storage driver
To configure Docker to use the overlay storage driver your Docker host must be
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. OverlayFS can operate on top of most supported Linux
filesystems. However, ext4 is currently recommended for use in production
environments.
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.
@ -216,7 +303,7 @@ OverlayFS. The procedure assumes that the Docker daemon is in a stopped state.
$ lsmod | grep overlay
overlay
3. Start the Docker daemon with the `overlay` storage driver.
3. Start the Docker daemon with the `overlay`/`overlay2` storage driver.
$ dockerd --storage-driver=overlay &
[1] 29403
@ -226,12 +313,12 @@ OverlayFS. The procedure assumes that the Docker daemon is in a stopped state.
<output truncated>
Alternatively, you can force the Docker daemon to automatically start with
the `overlay` driver by editing the Docker config file and adding the
`--storage-driver=overlay` flag to the `DOCKER_OPTS` line. Once this option
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` storage driver
4. Verify that the daemon is using the `overlay`/`overlay2` storage driver
$ docker info
Containers: 0
@ -244,22 +331,23 @@ OverlayFS. The procedure assumes that the Docker daemon is in a stopped state.
`extfs`. Multiple backing filesystems are supported but `extfs` (ext4) is
recommended for production use cases.
Your Docker host is now using the `overlay` 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"
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` driver 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` storage driver.
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` driver efficient with memory and a good
option for PaaS and other high density use cases.
- **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
@ -274,13 +362,14 @@ possible to incur far larger latencies if searching through many AUFS layers.
- **RPMs and Yum**. 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. Therefore, using `yum` inside of a
container on a Docker host using the `overlay` storage driver is unlikely to
work without implementing workarounds.
container on a Docker host using the `overlay`/`overlay2` storage drivers is
unlikely to work without implementing workarounds.
- **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.
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