spring-boot-demo/docker-images/docs/02-measuring-size.md

2. Measuring size

← 1. The variants · Index · Next: 3. Layered jars →

"Image size" means at least three different numbers:

Number	What it is	How it is measured here
on disk	uncompressed layers, what a node stores	docker image inspect -f '{{.Size}}' on the classic overlay2 store
pushed	compressed layer blobs + config, what a registry stores and a cold node downloads	sum of layers[].size + config.size from crane manifest
rebuild delta	the blobs a push actually uploads after a change	chapter 3

measure.sh pushes every variant to a throwaway registry (crane registry serve --address localhost:5000 - in-memory, no Docker Hub account needed) and reads the manifests back.

docker images disagrees with itself

Docker Engine 29 can use two image stores. The same eleven tarballs, loaded into each (image-store-size-difference.txt):

Image	containerd store	classic overlay2
eclipse-temurin:25-jre	496 MB	352 MB
gcr.io/distroless/java25-debian13:nonroot	305 MB	226 MB
paketobuildpacks/builder-noble-java-tiny	1.26 GB	868 MB

The containerd store's figure is larger across the board. Size comparisons from blog posts - or between two colleagues' laptops - are only comparable if they used the same store, and the containerd store is the default for new Docker installations. This project measures on the classic store because its number is the unpacked size alone.

The result

image-matrix.txt:

variant                on disk    pushed  layers user      shell  packages          ready ms   Started       RSS  JVM
fatjar-jdk                456M      178M       7 root      yes    115 (dpkg)            5362    3.932s  169.5MiB  Eclipse 25.0.4
fatjar-jre                377M      144M       7 root      yes    106 (dpkg)            5091    3.944s  182.1MiB  Eclipse 25.0.4
layered-jre               377M      144M      11 root      yes    106 (dpkg)            4272    3.356s  169.5MiB  Eclipse 25.0.4
layered-alpine            251M       98M      10 root      yes    44 (apk)              4634    3.775s  145.9MiB  Eclipse 25.0.4
layered-distroless        252M       97M      39 65532     no     25 (status.d)         4118    3.254s  164.3MiB  Eclipse 25.0.4.1
jlink-distroless          119M       76M      39 65532     no     24 (status.d)         4386     3.39s  165.7MiB  Eclipse 25.0.4
aot-cache                 440M      160M      12 root      yes    106 (dpkg)            2412    1.638s  174.6MiB  Eclipse 25.0.4
buildpacks                345M      125M      20 1002:1001 no     10 (status.d)         4726    3.615s  183.6MiB  BellSoft 25.0.4
jib                       377M      144M      10 root      yes    106 (dpkg)            4323    3.457s  162.5MiB  Eclipse 25.0.4

• ready ms is docker run to the first 200 from /actuator/health/readiness; Started is Spring Boot's own figure. Median of three on a 2-vCPU VM - treat differences under ~0.3 s as noise.
• packages counts OS packages from the image filesystem without running anything in it (dpkg status, distroless/Paketo status.d/, or apk), which works for images with no shell.
• Every Temurin-based variant runs as root: the official images set no USER.

A note on method: the first run of this table had distroless starting two seconds slower than everything else. A k3s image import was running on the same two CPUs at the time. Nothing else runs during measure.sh now.