Bloated image unique sizes after update to containerd image store in Docker 29

Description

After upgrading a Docker host with various Docker images to a fresh Docker 29 installation, we have suddendly lost 6 GB of available storage in the /var partition. Investigation shows significant increases in image unique sizes (2x or even 3x).

Rebuilding the images with the "containerd-snapshotter" feature disabled in daemon.json produces again the expected unique sizes, and the images occupy the same as with Docker 28.

Reproduce

1. Create Dockerfile.base with contents:

FROM oraclelinux:9
RUN dnf -y install openssh-server

2. Create Dockerfile with contents:

FROM mybase:latest
RUN dnf -y install sudo

3. Build both images in Docker 29 (fresh installation):

docker build -f Dockerfile-base -t mybase .
docker build -f Dockerfile -t myimage .

4. Check image sizes with docker system df --verbose:

REPOSITORY   TAG       IMAGE ID       CREATED          SIZE      SHARED SIZE   UNIQUE SIZE   CONTAINERS
myimage      latest    266eb647e381   20 minutes ago   965MB     576.6MB       388.1MB       0
mybase       latest    d5a5ad4b045d   21 minutes ago   927MB     576.6MB       350.1MB       0

5. Repeat the steps in either a) Docker 28, b) an existing Docker host upgraded to Docker 29, or c) a fresh Docker 29 installation with the containerd-snapshotter feature disabled. The image sizes are:

REPOSITORY   TAG       IMAGE ID       CREATED          SIZE      SHARED SIZE   UNIQUE SIZE   CONTAINERS
myimage      latest    8c21b1c4594a   57 minutes ago   589MB     561.5MB       27.74MB       0
mybase       latest    7f911452060c   58 minutes ago   562MB     561.5MB       0B            0

Expected behavior

Image sizes should be similar to those produced by the legacy image store.

docker version

Client: Docker Engine - Community
 Version:           29.0.2
 API version:       1.52
 Go version:        go1.25.4
 Git commit:        8108357
 Built:             Mon Nov 17 12:36:49 2025
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Engine - Community
 Engine:
  Version:          29.0.2
  API version:      1.52 (minimum version 1.44)
  Go version:       go1.25.4
  Git commit:       e9ff10b
  Built:            Mon Nov 17 12:33:39 2025
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          v2.1.5
  GitCommit:        fcd43222d6b07379a4be9786bda52438f0dd16a1
 runc:
  Version:          1.3.3
  GitCommit:        v1.3.3-0-gd842d771
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

docker info

Client: Docker Engine - Community
 Version:    29.0.2
 Context:    default
 Debug Mode: false
 Plugins:
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.30.0
    Path:     /usr/libexec/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.40.3
    Path:     /usr/libexec/docker/cli-plugins/docker-compose

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 2
 Server Version: 29.0.2
 Storage Driver: overlayfs
  driver-type: io.containerd.snapshotter.v1
 Logging Driver: local
 Cgroup Driver: systemd
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: fcd43222d6b07379a4be9786bda52438f0dd16a1
 runc version: v1.3.3-0-gd842d771
 init version: de40ad0
 Security Options:
  seccomp
   Profile: builtin
  cgroupns
 Kernel Version: 5.15.0-314.193.5.4.el9uek.x86_64
 Operating System: Oracle Linux Server 9.6
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 7.31GiB
 Name: <redacted>
 ID: 162cfa2a-a82e-4245-ae9a-9ab1b05cdbc8
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 Experimental: false
 Insecure Registries:
  ::1/128
  127.0.0.0/8
 Registry Mirrors:
  <redacted>
 Live Restore Enabled: false
 Firewall Backend: iptables

Additional Info

No response

Start by reproducing the two-image build with Docker 29 and the containerd-snapshotter feature enabled and disabled, then compare docker system df --verbose output. Trace the image-size and unique-size reporting path involved in the containerd image store. Done means the reproduced images report storage usage comparable to Docker 28 or the legacy image store.