Does file touch create a copy in the docker container layer?

Question

What about making only a file timestamp change (i.e. touch'ed) in a docker container? Would UFS (or the varianet used) create a file copy in the docker container layer or is it able to handle this scenario more efficiently?

Looking to pinpoint information about the effects of file touch in context of docker, and how to best manage this scenario to reduce cumulative file storage size footprint over the layers.

Answer 1

This is an easy question to answer experimentally. Start a container:

docker run -it ubuntu

Touch something and exit the container:

root@914f5453af3c:/# touch /bin/ls
root@914f5453af3c:/# exit

Save the container as a new image:

$ docker commit 914f5453af3c testimage

Export the image layers to a local directory:

$ mkdir testimage
$ docker save testimage | tar -C testimage -xf-

This will give you a directory that looks something like:

$ ls testimage
15c6fddb70d1d281b7c20cdd5a54be3379c4c282a3e9ba2ae27c79a655ca9ed6
46bda337e95a0163468d9daa3a13ba104507f68186a09cdb2892b20030aeb530
63c22c7d4e3cd1a04ada1a24dd84837c1ce0c445bb48b75ed45f84250146459f
6d1bde67c5695c66882fd71951376e176ba908fc9a560bd8eeb07c4b4797da89
7c05062524dd680596681fb193b3096fcd775da2c0e7e66e0ad580850ee3a973.json
96ab7e8335a8dff5ed0144a46ddbe6ab6c1e76011670cb54efc515c46ea6b09b
fbe2d84b180c3f84eb15754cf95e1a6359a84ed3c6e2f6869f0a2015343611fc
manifest.json
repositories

Look at repositories to find the topmost layer:

$  cat testimage/repositories

{"testimage":{"latest":"6d1bde67c5695c66882fd71951376e176ba908fc9a560bd8eeb07c4b4797da89"}}

Look at the contents of that layer:

$ tar -tf testimage/6d1bde67c5695c66882fd71951376e176ba908fc9a560bd8eeb07c4b4797da89/
layer.tar
bin/
bin/ls
root/
root/.bash_history

And there you can see that the topmost layer has a copy of /bin/ls as a result of running touch.