Docker for Mac: add page on disk space

docker-for-mac/space.md

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+---
+description: Disk utilization
+keywords: mac, disk
+title: Disk utilization in Docker for Mac
+---
+
+Docker for Mac stores Linux containers and images in a single, large "disk image" file
+in the Mac filesystem. This is different from Docker on Linux, which usually stores containers
+and images in the `/var/lib/docker` directory.
+
+## Where is the "disk image" file?
+
+To locate the "disk image" file, first select the whale menu icon and then select
+**Preferences...**. When the **Preferences...** window is displayed, select **Disk** and then **Reveal in Finder**:
+
+![Disk preferences](images/settings-disk.png)
+
+The **Preferences...** window shows how much actual disk space the "disk image" file is consuming.
+In this example, the "disk image" file is consuming 2.4 GB out of a maximum of 64 GB.
+
+Note that other tools might display space usage of the file in terms of the maximum file size, not the actual file size.
+
+## If the file is too big
+
+If the file is too big, you can
+- move it to a bigger drive,
+- delete unnecessary containers and images, or
+- reduce the maximum allowable size of the file.
+
+### To move the file to a bigger drive
+
+To move the file, open the **Preferences...** menu, select **Disk**  and then select
+on **Move disk image**. Do not move the file directly in the finder or Docker for Mac will
+lose track of it.
+
+### To delete unnecessary containers and images
+
+To check whether you have too many unnecessary containers and images:
+
+If your client and daemon API are version 1.25 or later (use the docker version command on the client to check your client and daemon API versions.), you can display detailed space usage information with:
+
+```
+docker system df -v
+```
+
+Alternatively, you can list images with:
+```bash
+$ docker image ls
+```
+and then list containers with:
+```bash
+$ docker container ls -a
+```
+
+If there are lots of unneeded objects, try the command
+```bash
+$ docker system prune
+```
+This removes all stopped containers, unused networks, dangling images, and build cache.
+
+Note that it might take a few minutes before space becomes free on the host, depending
+on what format the "disk image" file is in:
+- If the file is named `Docker.raw`: space on the host should be reclaimed within a few
+  seconds.
+- If the file is named `Docker.qcow2`: space will be freed by a background process after
+  a few minutes.
+
+Note that space is only freed when images are deleted. Space is not freed automatically
+when files are deleted inside running containers. To trigger a space reclamation at any
+point, use the command:
+
+```
+$ docker run --privileged --pid=host justincormack/nsenter1 /sbin/fstrim /var/lib/docker
+```
+
+Note that many tools will report the maximum file size, not the actual file size.
+To query the actual size of the file on the host from a terminal, use:
+```bash
+$ cd ~/Library/Containers/com.docker.docker/Data
+$ cd vms/0   # or com.docker.driver.amd64-linux
+$ ls -klsh Docker.raw
+2333548 -rw-r--r--@ 1 akim  staff    64G Dec 13 17:42 Docker.raw
+```
+In this example, the actual size of the disk is `2333548` KB, whereas the maximum size
+of the disk is `64` GB.
+
+### To reduce the maximum size of the file
+
+To reduce the maximum size of the file, first click on the whale menu and then click on
+"Preferences...". When the preferences window appears, click on the "Disk" icon.
+The preferences window contains a slider which allows the maximum disk size to be set.
+**Warning**: If the maximum size is reduced, the current file will be deleted and, therefore, all
+containers and images will be lost.
+