If images in Docker are untagged, are they used at all or just orphans?

Question

Running the commands docker images and docker images -a results in the following outputs:

$ docker images
REPOSITORY           TAG        IMAGE ID            CREATED         VIRTUAL SIZE
ubuntu               14.04      9cbaf023786c        2 days ago      192.8 MB
$ docker images -a
REPOSITORY           TAG        IMAGE ID            CREATED         VIRTUAL SIZE
ubuntu               14.04      9cbaf023786c        2 days ago      192.8 MB
<none>               <none>     03db2b23cf03        2 days ago      192.8 MB
<none>               <none>     8f321fc43180        2 days ago      192.8 MB
<none>               <none>     6a459d727ebb        2 days ago      192.8 MB
<none>               <none>     2dcbbf65536c        2 days ago      192.8 MB
<none>               <none>     97fd97495e49        2 days ago      192.6 MB

Are the images tagged <none> of any importance? If not: why do they come with the tagged images I pulled? Do the sizes add up or are they just a repition? If so, can I delete them without any effect on my work?

Answer 1

The image files are independent, and combine via unionfs magic to form a running container. The images you care about are often tagged with memorable names. You can delete the unused images, i.e. those not contributing to any image you care about. I do it this way in bash:

function docker_rm_unnamed_images {
  sudo docker rmi $(sudo docker images | grep '^<none>' | awk '{print $3}')
}

You can think of a docker image as a stack of 'layers'. Each Dockerfile command adds an additional layer to the image. It's important to realize that each of those commands creates a separate image file. So the Dockerfile

FROM foo
RUN a
RUN b
RUN c

would be a stack of

image=1 (possibly pulled from the foo registry)
image=2 (after applying a to image 1)
image=3 (after applying b to image 2)
image=4 (after applying c to image 3)

It is likely that the foo image was composed of multiple other layers, so your final image is a stack of 4 or more images. Each of those image files lives in your docker image registry. Most of them are unnamed, because they correspond to a RUN command, for example. Each of the image files 1-4 are probably fairly small (unless they correspond to a yum install p1 .. p100 for example). Together they make up the file system of the container that you ultimately run.