Anatomy of the Docker for Mac virtual disk

Experimenting with Docker for Mac (beta) I wanted to look inside the virtual machine running Docker and reclaim unused disk space.

First of all -- it stores the virtual disk under ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2.

In the next step I have copied this file into the folder where a Vagrant Linux VM run, listed content, found a running Docker container and shrunk virtual disk:

	# Show some info about the virtual disk
	mymac> qemu-img check ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2  ✓  10118
	No errors were found on the image.
	54968/1048576 = 5.24% allocated, 0.95% fragmented, 0.00% compressed clusters
	Image end offset: 3606183936
	mymac> qemu-img info ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2  ✓  10138
	image: ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2
	file format: qcow2
	virtual size: 64G (68719476736 bytes)
	disk size: 3.4G
	cluster_size: 65536
	Format specific information:
	compat: 1.1
	lazy refcounts: true
	refcount bits: 16
	corrupt: false
	# Copy and compress virtual disk to Linux VM folder
	qemu-img convert -c -O qcow2 ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/Docker.qcow2 dockermac.qcow2
	# Login into Linux VM
	mymac>vagrant ssh
	[vagrant@centos7-adb ~]$ ll /vagrant
	total 1065220
	-rw-r--r--. 1 qemu qemu 1090781184 Jun 10 12:33 dockermac.qcow2
	-rw-r--r--. 1 vagrant vagrant 2137 Jun 10 12:08 Vagrantfile
	# Install libguestfs
	[vagrant@centos7-adb ~]$ sudo yum install libguestfs-tools
	# Start libguestfs shell
	[vagrant@centos7-adb ~]$ sudo guestfish
	><fs> add /vagrant/dockermac.qcow2 # add a copy of Docker for Mac virtual disk
	><fs> run
	100% ⟦▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒⟧ 00:00
	><fs> list-filesystems
	/dev/sda1: swap
	/dev/sda2: ext4
	><fs> mount /dev/sda2 / # mount virtual disk under Linux VM
	><fs> ll /
	total 52
	drwxr-xr-x 10 root root 4096 Jun 9 13:29 .
	drwxr-xr-x 19 root root 4096 Jun 10 16:17 ..
	drwxr-xr-x 4 root root 4096 May 20 20:54 cache
	drwxr-xr-x 2 root root 4096 May 6 06:00 empty
	drwxr-xr-x 2 root root 4096 May 20 20:54 git
	drwxr-xr-x 8 root root 4096 Jun 9 13:29 lib
	lrwxrwxrwx 1 root root 9 Jun 9 13:29 lock -> /run/lock
	drwxr-xr-x 3 root root 4096 Jun 9 13:29 log
	drwx------ 2 root root 16384 Jun 9 13:29 lost+found
	lrwxrwxrwx 1 root root 4 Jun 9 13:29 run -> /run
	drwxr-xr-x 3 root root 4096 May 6 06:00 spool
	drwxrwxrwt 2 root root 4096 May 6 06:00 tmp
	---------- 1 root root 0 Jun 9 13:29 transfused_start.log
	# found a container running in Mac
	><fs> ll /lib/docker/containers/45a946cb64d29f7aea8789c710093d494ae07d6bb9dc256ddb5e70e1992be95d/
	total 1016
	drwx------ 3 root root 4096 Jun 9 14:14 .
	drwx------ 3 root root 4096 Jun 10 09:46 ..
	-rw-r----- 1 root root 1000621 Jun 10 10:06 45a946cb64d29f7aea8789c710093d494ae07d6bb9dc256ddb5e70e1992be95d-json.log
	-rw-r--r-- 1 root root 3337 Jun 9 14:14 config.v2.json
	-rw-r--r-- 1 root root 1101 Jun 9 14:14 hostconfig.json
	-rw-r--r-- 1 root root 13 Jun 9 14:14 hostname
	-rw-r--r-- 1 root root 174 Jun 9 14:14 hosts
	-rw-r--r-- 1 root root 37 Jun 9 14:14 resolv.conf
	-rw------- 1 root root 71 Jun 9 14:14 resolv.conf.hash
	drwx------ 2 root root 4096 Jun 9 14:14 shm
	[vagrant@centos7-adb ~]$ exit
	# Now if you want to shrink virtual disk to save space (e.g. after you cleaned up obsolete images and containers)
	# Stop running Docker daemon
	mymac> cd ~/Library/Containers/com.docker.docker/Data/com.docker.driver.amd64-linux/
	mymac> mv Docker.qcow2 Docker.qcow2.backup
	mymac> qemu-img convert -O qcow2 Docker.qcow2.backup Docker.qcow2
	mymac> qemu-img check Docker.qcow2.backup  ✓  10162
	No errors were found on the image.
	54969/1048576 = 5.24% allocated, 0.96% fragmented, 0.00% compressed clusters
	Image end offset: 3606249472
	mymac> qemu-img check Docker.qcow2  ✓  10163
	No errors were found on the image.
	39636/1048576 = 3.78% allocated, 0.01% fragmented, 0.00% compressed clusters
	Image end offset: 2601385984
	# Start your Docker for mac
	mymac> /Applications/Docker.app/Contents/Resources/bin/docker-diagnose  ✓  10165
	OS X: version 10.11.5 (build: 15F34)
	Docker.app: version: v1.11.1-beta14.5.m
	Running diagnostic tests:
	[OK] docker-cli
	[OK] Moby booted
	[OK] driver.amd64-linux
	[OK] vmnetd
	[OK] osxfs
	[OK] db
	[OK] slirp
	[OK] menubar
	[OK] environment
	[OK] Docker
	[OK] VT-x
	Docker logs are being collected into /tmp/20160610-205107.tar.gz
	Most specific failure is: No error was detected
	# All good!

view raw VM session.sh hosted with ❤ by GitHub

Not a lot of science, nevertheless -- learned a new tool and now know where to search and what to do if my local Docker is getting occupy too much disk space.

PS: Special thanks to Richard WM Jones for useful tips about guestfs tools and James Coyle for an article Reclaim disk space from a sparse image file (qcow2/vmdk),