![]() ![]() They barely take up any space and are more for record keeping of what settings apply to the image you are running. There are also layers being added for changes to the image meta-data, like the CMD line. When you build one image, you inherit all the layers of the image you specify in the FROM line. Of note, there are two layers at the bottom that are fairly old. Here's the history for the above image: IMAGE CREATED CREATED BY SIZE COMMENTĪ81cfb93008c 4 seconds ago /bin/sh -c #(nop) CMD ["/bin/sh" "-c" "ls -… 0Bį36265598aef 5 seconds ago /bin/sh -c rm /data/one 0BĬ79aff033b1c 7 seconds ago /bin/sh -c chmod -R 0777 /data 2.1MBī821dfe9ea38 10 seconds ago /bin/sh -c dd if=/dev/zero bs=1024 count=102… 1.05MBĪ5602b8e8c69 13 seconds ago /bin/sh -c chmod -R 0777 /data 1.05MBĠ8ec3c707b11 15 seconds ago /bin/sh -c dd if=/dev/zero bs=1024 count=102… 1.05MBĮd27832cb6c7 18 seconds ago /bin/sh -c mkdir /data 0BĢ2c2dd5ee85d 2 weeks ago /bin/sh -c #(nop) CMD 0B You can also run a docker image inspect on an image and see the list of layers under the RootFS section. You can see the commands that goes into creating the layers of an existing image with the docker history command. The deleted /data/one file is still in the previous layers, 3 times in fact, and will be copied over the network and stored on disk when you pull the image. The entire file is in each layer when there's an add or change, so each of those chmod commands, despite just changing a permission bit, results in the entire file being copied into the next layer. Here's the TL DR partĮach of these container filesystem diffs above goes into one "layer" that gets assembled when you run the image as a container. $ docker diff 504c6e9b6637 # rm /data/oneĮach line prefixed with an A is adding the file, the C indicates a change to an existing file, and the D indicates a delete. $ docker diff 038bd2bc5aea # chmod -R 0777 /data $ docker diff f1b72db3bfaa # dd if=/dev/zero bs=1024 count=1024 of=/data/one ![]() If you run a docker diff on each of those container id's, you'll see what files were created in those containers: $ docker diff 04c5fa1360b0 # mkdir /data In the output, you'll see each of the running commands happen in a temporary container that we now keep instead of automatically deleting. Lets build the image with an extra flag to save the temporary containers: docker image build -rm=false. # and now this cleans up that downloaded source codeĮach of those dd commands outputs a 1M file to the disk. RUN dd if=/dev/zero bs=1024 count=1024 of=/data/two RUN dd if=/dev/zero bs=1024 count=1024 of=/data/one # imagine this is downloading source code Lets take a contrived example Dockerfile: FROM busybox Examining layers of your own build with docker diff They make the most sense to me with an example. Because layers are intermediate images, if you make a change to your Dockerfile, docker will rebuild only the layer that was changed and the ones after that. The concept of layers comes in handy at the time of building images. Then, we tell docker to run bundle exec puma (which boots up the rails server). ![]() ![]() We add another layer on top of our starting image, setting the environment variable RAILS_ENV with the ENV command. You can think of it as staging changes when you're using git: You add a file's change, then another one, then another one.Ĭonsider the following Dockerfile: FROM rails:onbuildįirst, we choose a starting image: rails:onbuild, which in turn has many layers. Every command you specify ( FROM, RUN, COPY, etc.) in your Dockerfile causes the previous image to change, thus creating a new layer. I might be late, but here's my 10 cents (complementing ashishjain's answer):īasically, a layer, or image layer is a change on an image, or an intermediate image. ![]()
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