Merge pull request #1100 from djs55/docker-for-mac-trim

docker-for-mac: add a FAQ about reducing the qcow2 size

docker-for-mac/faqs.md

@@ -136,6 +136,43 @@ For a complete explanation of how to do this, see the blog post [Adding Self-sig
 to Docker & Docker for
 Mac](http://container-solutions.com/adding-self-signed-registry-certs-docker-mac/).
 
+### How do I reduce the size of Docker.qcow2?
+
+By default Docker for Mac stores containers and images in a file
+`~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2`.
+This file grows on-demand up to a default maximum file size of 64GiB.
+
+In Docker 1.12 the only way to free space on the host is to delete
+this file and restart the app. Unfortunately this removes all images
+and containers.
+
+In Docker 1.13 there is preliminary support for "TRIM" to non-destructively
+free space on the host. First free space within the `Docker.qcow2` by
+removing unneeded containers and images using
+
+- `docker ps -a`: list all containers
+- `docker image ls`: list all images
+- `docker system prune`: (new in 1.13): deletes all stopped containers, all
+  volumes not used by at least one container and all images without at least one
+  referring container.
+
+Note the `Docker.qcow2` will not shrink in size immediately.
+In 1.13 a background `cron` job runs `fstrim` every 15 minutes.
+If the space needs to be reclaimed sooner, run this command:
+
+```
+docker run --rm -it --privileged --pid=host walkerlee/nsenter -t 1 -m -u -i -n fstrim /var
+```
+
+Once the `fstrim` has completed, restart the app. When the app shuts down it
+will compact the file and the space will be freed. Note the app will
+take longer to restart than usual because it must wait for the
+compaction to complete.
+
+For background conversation thread on this, see [Docker.qcow2 never shrinks
+..](https://github.com/docker/for-mac/issues/371) on Docker for Mac GitHub
+issues.
+
 ### What are system requirements for Docker for Mac?
 
 Note that you need a Mac that supports hardware virtualization, which is most non ancient ones; i.e., use macOS `10.10.3+` or `10.11` (macOS Yosemite or macOS El Capitan). See also "What to know before you install" in [Getting Started](index.md).