Document that overlay2 is now the default for ubuntu (#5050)

engine/installation/linux/docker-ce/ubuntu.md

@@ -51,7 +51,20 @@ It's OK if `apt-get` reports that none of these packages are installed.
 The contents of `/var/lib/docker/`, including images, containers, volumes, and
 networks, are preserved. The Docker CE package is now called `docker-ce`.
 
-### Recommended extra packages for Trusty 14.04
+### If you need to use aufs
+
+Docker CE now uses the `overlay2` storage driver by default, and it is
+recommended that you use it instead of `aufs`. If you need to use `aufs`, you
+will need to do additional preparation.
+
+#### Xenial 16.04 and newer
+
+For Ubuntu 16.04 and higher, the Linux kernel includes support for OverlayFS,
+and Docker CE will use the `overlay2` storage driver by default. If you need
+to use `aufs` instead, you need to configure it manually.
+See [aufs](/engine/userguide/storagedriver/aufs-driver.md)
+
+#### Trusty 14.04
 
 Unless you have a strong reason not to, install the
 `linux-image-extra-*` packages, which allow Docker to use the `aufs` storage
@@ -65,9 +78,6 @@ $ sudo apt-get install \
     linux-image-extra-virtual
 ```
 
-For Ubuntu 16.04 and higher, the Linux kernel includes support for OverlayFS,
-and Docker CE will use the `overlay2` storage driver by default.
-
 ## Install Docker CE
 
 You can install Docker CE in different ways, depending on your needs:

engine/userguide/storagedriver/aufs-driver.md

@@ -4,20 +4,14 @@ keywords: 'container, storage, driver, AUFS '
 title: Use the AUFS storage driver
 ---
 
-AUFS a *union filesystem*. The `aufs` storage driver is the default storage
-driver used for managing images and layers on Docker for Ubuntu, and for
-Debian versions prior to Stretch. For Debian Stretch,
-[overlay2](overlayfs-driver.md) is the default.
+AUFS is a *union filesystem*. The `aufs` storage driver was previously the default
+storage driver used for managing images and layers on Docker for Ubuntu, and for
+Debian versions prior to Stretch. If your Linux kernel is version 4.0 or higher,
+and you use Docker CE, consider using the newer
+[overlay2](overlayfs-driver.md){: target="_blank" class="_" }, which has
+potential performance advantages over the `aufs` storage driver.
 
-AUFS is the most mature storage driver used by Docker. It provides fast
-container startup time, as well as efficient usage of memory and storage.
-
-If your Linux kernel is version 4.0 or higher, and you use Docker CE, consider
-using the newer [overlay2](overlayfs-driver.md){: target="_blank" class="_" },
-which has potential performance advantages over the `aufs` storage driver.
-
-> **Note**: Even though AUFS is used by default if it is present in the Linux
-> kernel, it is not supported on some distributions and Docker editions. See
+> **Note**: AUFS is not supported on some distributions and Docker editions. See
 > [Prerequisites](#prerequisites) > for more information about supported
 > platforms, and see also
 > [the order of preferences for storage drivers](selectadriver.md#storage-driver-order).

engine/userguide/storagedriver/selectadriver.md

@@ -22,9 +22,6 @@ this decision, there are three high-level factors to consider:
   explicitly configured, assuming that the prerequisites for that storage driver
   are met:
 
-  - If `aufs` is available, default to it, because it is the oldest storage
-    driver. However, it is not universally available.
-
   - If possible, the storage driver with the least amount of configuration is
     used, such as `btrfs` or `zfs`. Each of these relies on the backing
     filesystem being configured correctly.
@@ -33,20 +30,22 @@ this decision, there are three high-level factors to consider:
     and stability in the most usual scenarios.
 
     - `overlay2` is preferred, followed by `overlay`. Neither of these requires
-      extra configuration.
+      extra configuration. `overlay2` is the default choice for Docker CE.
 
     - `devicemapper` is next, but requires `direct-lvm` for production
       environments, because `loopback-lvm`, while zero-configuration, has very
       poor performance.
 
-  The selection order is defined in Docker's source code. You can see the order
-  for Docker 17.03 by looking at
-  [the source code](https://github.com/moby/moby/blob/v17.03.1-ce/daemon/graphdriver/driver_linux.go#L54-L63).
-  For a different Docker version, change the URL to that version.
+  The selection order is defined in Moby's source code. You can see the order
+  by looking at
+  [the source code](https://github.com/docker/docker-ce/blob/master/components/engine/daemon/graphdriver/driver_linux.go#L54-L63).
+  The order may vary slightly by version. Select the branch for the version you
+  use to verify the storage driver order.
   {: id="storage-driver-order" }
 
 - Your choice may be limited by your Docker edition, operating system, and
-  distribution. For instance, `aufs` is only supported on Ubuntu and Debian,
+  distribution. For instance, `aufs` is only supported on Ubuntu and Debian, and
+  may require extra packages to be installed,
   while `btrfs` is only supported on SLES, which is only supported with Docker
   EE. See
   [Support storage drivers per Linux distribution](#supported-storage-drivers-per-linux-distribution).
@@ -91,6 +90,15 @@ configurations work on recent versions of the Linux distribution:
 | Docker CE on CentOS | `devicemapper`, `vfs`                                                                                 |
 | Docker CE on Fedora | `devicemapper`, `overlay2` (Fedora 26 or later, experimental), `overlay` (experimental), `vfs`        |
 
+When possible, `overlay2` is the recommended storage driver. When installing
+Docker for the first time, `overlay2` is used by default. Previously, `aufs` was
+used by default when available, but this is no longer the case. If you want to
+use `aufs` on new installations going forward, you need to explicitly configure
+it, and you may need to install extra packages, such as `linux-image-extra`.
+See [aufs](aufs-driver.md).
+
+On existing installations using `aufs`, it will continue to be used.
+
 When in doubt, the best all-around configuration is to use a modern Linux
 distribution with a kernel that supports the `overlay2` storage driver, and to
 use Docker volumes for write-heavy workloads instead of relying on writing data