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Upgrade to 20.10 writes not-removable data into /buildkit/content/blobs/sha256 #42010

Description

@entasisj

Description

After upgrade from 19.03.x to 20.10.x we noticed that a raising amount of GB of disk space was occupied within folder "docker/buildkit/content/blobs/sha256". This only happens when building with "DOCKER_BUILDKIT=1". Although we are building with "--no-cache" the space gets occupied.
We tried to cleanup calling "docker system prune -af" and "docker builder prune -af" or "docker buildx prune -af". Nothing removed the data.
Calling "docker system df" or "docker buildx du" afterwards reported 0B of any occupied space which is surprising since there were more than 30 GB occupied in docker/buildkit/content/blobs/sha256.

Steps to reproduce the issue:

  1. Upgrade docker-ce to 20.10.x.
  2. Check occupied space in 'buildkit' subfolder: "sudo du -h -d 1 /var/lib/docker/buildkit"
  3. Build an image deriving from CUDA development image: "FROM nvidia/cuda:11.0-devel-ubuntu20.04" with "DOCKER_BUILDKIT=1 docker build --no-cache"
  4. Check increased occupied space in 'buildkit' subfolder: "sudo du -h -d 1 /srv/docker/buildkit"
  5. Clean up: "sudo docker prune -af"
  6. Check done cleanup: "sudo docker system df"
  7. Check that there is no change in occupied space in 'buildkit' subfolder: "sudo du -h -d 1 /srv/docker/buildkit"

Describe the results you received:

The occupied space in /var/lib/docker/buildkit was not freed.

Describe the results you expected:

Occupied space in /var/lib/docker/buildkit was freed.

Additional information you deem important (e.g. issue happens only occasionally):
Issue always happens.

Output of docker version:

Client: Docker Engine - Community
 Version:           20.10.3
 API version:       1.41
 Go version:        go1.13.15
 Git commit:        48d30b5
 Built:             Fri Jan 29 14:33:21 2021
 OS/Arch:           linux/amd64
 Context:           default
 Experimental:      true

Server: Docker Engine - Community
 Engine:
  Version:          20.10.3
  API version:      1.41 (minimum version 1.12)
  Go version:       go1.13.15
  Git commit:       46229ca
  Built:            Fri Jan 29 14:31:32 2021
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.4.3
  GitCommit:        269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc:
  Version:          1.0.0-rc92
  GitCommit:        ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

Output of docker info:

Client:
 Context:    default
 Debug Mode: false
 Plugins:
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)

Server:
 Containers: 0
  Running: 0
  Paused: 0
  Stopped: 0
 Images: 0
 Server Version: 20.10.3
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 io.containerd.runtime.v1.linux runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 269548fa27e0089a8b8278fc4fc781d7f65a939b
 runc version: ff819c7e9184c13b7c2607fe6c30ae19403a7aff
 init version: de40ad0
 Security Options:
  apparmor
  seccomp
   Profile: default
 Kernel Version: 5.4.0-65-generic
 Operating System: Ubuntu 20.04.2 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 2
 Total Memory: 1.941GiB
 Name: friday-agent-1
 ID: TL7W:VGHA:PUTC:W3TV:R5MT:I4PL:QSR5:SMO3:LMJP:TLYU:DQCI:PVSL
 Docker Root Dir: /srv/docker
 Debug Mode: false
 Username: fraunhofermevis
 Registry: https://index.docker.io/v1/
 Labels:
 Experimental: false
 Insecure Registries:
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: No swap limit support
WARNING: No blkio weight support
WARNING: No blkio weight_device support

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