partialdockersave

This is a alternative of docker save which you can only save layers not exists in specified image.

• What's the purpose of this tool

Sometimes, we need to use docker save to dump the image and then transfer it to an enviroment without internet. A problem could be met that the result filesize is always too big.

In fact, we may already have an old image in the dest server which is almost the same with new image. This tool helps you to save only the layers that doesn't exist in the old images.

Install

go install github.com/penfree/partialdockersave@latest

Usage

NAME:
   Save docker image without layers in `exclude`

USAGE:
   partialdockersave [global options] command [command options] [arguments...]

COMMANDS:
     help, h  Shows a list of commands or help for one command

GLOBAL OPTIONS:
   --image value, -i value    The image to save
   --exclude value, -e value  The existing image that does not need to export
   --output value, -o value   The output tar.gz file (default: "image.tgz")
   --help, -h                 show help
   --version, -v              print the version

Example

# multiple -i & -e can be specified to include or exclude
partialdockersave -i image1 -i image2 -e imagetoexclude -o result.tgz

How it works

docker save can give us a tar file with one directory for each layer of the image. The key is to know which directory is not needed.

• How is the directory name generated?

We can find from https://github.com/moby/moby/blob/master/image/tarexport/save.go that the directory name is encoded from layer meta data. Sothe same thing can be done from infomation got from docker image inspect, then filter them when saving image.

Unlike other tools (Ex: undocker, dive), they should do a real save to get the tar file of all images, and then extract manifest from the tar file, it could be a bit slow especially when multiple images need to be saved at the same time. This tool only compares the diff layer from docker image inspect, thus brings no more io operation than normal docker save.

Future works

Other than raw images, we can do some work to analyze images in helm chart and compute the increment diff of two helm chart. This can be done by implement ImageLike interface for helm charts.

Known issues

• Only tested on Ubuntu 16.04, docker 19.03.8.
• The latest layer id computed by this tool is not same with real layer id, it may cause that the latest layer will always be saved. But it's not a big issue, because you doesn't need to compare two identical image.