Diffrence between real and showed by docker system df disc usage - docker

Question

I have docker in version 18.03.1-ce that support command docker system df. His output:
Images space usage:
REPOSITORY TAG IMAGE ID CREATED ago SIZE SHARED SIZE UNIQUE SiZE CONTAINERS
registry.gitlab.com/precisesale/app latest b7833546c2cf About an hour ago ago 252.1MB 123.8MB 128.4MB 1
healthdiary/app latest 565c6d3906e6 2 days ago ago 312.2MB 123.8MB 188.4MB 1
mongo latest f93ff881751f 5 days ago ago 367.6MB 0B 367.6MB 2
nginx latest b175e7467d66 6 weeks ago ago 108.9MB 0B 108.9MB 1
jwilder/docker-gen latest 8959ee34c769 2 months ago ago 19.91MB 4.148MB 15.77MB 1
jrcs/letsencrypt-nginx-proxy-companion latest 17939ceb7a52 2 months ago ago 86.86MB 4.148MB 82.71MB 1
Containers space usage:
CONTAINER ID IMAGE COMMAND LOCAL VOLUMES SIZE CREATED ago STATUS NAMES
c20dc3438552 healthdiary/app "./entrypoint.sh nod…" 0 0B 8 minutes ago ago Up 8 minutes healthdiary_app_1
bf8c4307dcbb mongo:latest "docker-entrypoint.s…" 1 0B 8 minutes ago ago Up 8 minutes healthdiary_mongo_1
47fced8d18fe registry.gitlab.com/precisesale/app "./entrypoint.sh nod…" 0 0B 9 minutes ago ago Up 9 minutes precisesale_app_1
597d97d5c1fa mongo:latest "docker-entrypoint.s…" 1 0B 9 minutes ago ago Up 9 minutes precisesale_db_1
b5bb14faa910 jwilder/docker-gen "/usr/local/bin/dock…" 0 0B 7 hours ago ago Up 19 minutes nginx-gen
8eee2bee084a nginx "nginx -g 'daemon of…" 0 2B 7 hours ago ago Up 19 minutes nginx-web
6b8b0cd5d938 jrcs/letsencrypt-nginx-proxy-companion "/bin/bash /app/entr…" 0 1.66kB 7 hours ago ago Up 19 minutes nginx-letsencrypt
Local Volumes space usage:
VOLUME NAME LINKS SIZE
0a40fac6ca98e776dad972c8193362a51a485b3305979e58996545d97310a3c7 1 0B
929b0b88849ad4d390efd4666e6a0e5f82e0e6dd34f7a09f609de90b190e6148 1 0B
Build cache usage: 0B
Even if I do not take into account savings from shared space from two first containers summary size is 1147.5 MB
But if I measure size of docker overlay2 on disc by du I get
du -hs /var/lib/docker/overlay2/
2.7G /var/lib/docker/overlay2/
Where is reason of difference in size of containers measured by docker system df and du?

Answer 1

I was wondering the same thing some time ago.
It’s not a bug, it’s a feature :-)
du -sh /var/lib/docker/overlay2
is not showing objective value because merge folders have been mounted using overlay driver and du output is not actual disk allocation size.
You can see the actual disk allocation size by examining only diff folders like:
du -shc /var/lib/docker/overlay2/*/diff
You can test this in your environment like this:
run
df -h /dev/sd*
du -shc /var/lib/docker/overlay2/*/diff
du -sh /var/lib/docker/overlay2
Now start 20 centos containers and observe what has change:
for i in {1..20}; do docker run -itd centos bash; done
df -h /dev/sd*
du -shc /var/lib/docker/overlay2/*/diff
du -sh /var/lib/docker/overlay2
You can see that the actual disk allocation (df command) is just cca 200MB more than before, but “du” on whole folder outputs 4.2G allocation.
“du” on “diff” folders shows 212M what is correct.
This is how Docker works and what makes it great!