docker system prune -a isn't regaining reclaimable space

[alexanderadam]

Description

I'm not quite sure whether this is related to #40152, #21925, #38848 or one of the other space related issues (#942, #33775, #40036, #40412) but on one of our systems, Docker is reporting reclaimable space but won't reclaim it.

Steps to reproduce the issue:

Describe the results you received:

Total reclaimed space: 0B

Describe the results you expected:

Something like Total reclaimed space: 1.838GB

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

Output of docker version:

Docker version 19.03.12, build 48a66213fe

Output of docker info:

Click to expand

Client:
 Debug Mode: false

Server:
 Containers: 19
  Running: 19
  Paused: 0
  Stopped: 0
 Images: 12
 Server Version: 19.03.12
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local logentries splunk syslog
 Swarm: active
  NodeID: 0xbsa8su0qc5j2egervoy1rvv
  Is Manager: true
  ClusterID: y19v5dk1dt8juzhudbzh34wm2
  Managers: 1
  Nodes: 1
  Default Address Pool: 10.0.0.0/8  
  SubnetSize: 24
  Data Path Port: 4789
  Orchestration:
   Task History Retention Limit: 5
  Raft:
   Snapshot Interval: 10000
   Number of Old Snapshots to Retain: 0
   Heartbeat Tick: 1
   Election Tick: 10
  Dispatcher:
   Heartbeat Period: 5 seconds
  CA Configuration:
   Expiry Duration: 3 months
   Force Rotate: 0
  Autolock Managers: false
  Root Rotation In Progress: false
  Node Address: 5.148.177.102
  Manager Addresses:
   5.148.177.102:2377
 Runtimes: runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 7ad184331fa3e55e52b890ea95e65ba581ae3429
 runc version: dc9208a3303feef5b3839f4323d9beb36df0a9dd
 init version: fec3683
 Security Options:
  apparmor
  seccomp
   Profile: default
  userns
 Kernel Version: 4.15.0-112-generic
 Operating System: Ubuntu 18.04.5 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 4
 Total Memory: 2.929GiB
 Name: somemachine
 ID: BKAE:GMLF:B4OX:5A6A:LS6S:N34G:4LI7:6UQT:DZCQ:PY56:SIQW:IFPW
 Docker Root Dir: /var/lib/docker/4099.500000
 Debug Mode: false
 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

Additional environment details (AWS, VirtualBox, physical, etc.):

It's a customer's machine — I believe it's still a physical one (could also be a VM). I will get the info in case it's really relevant here.

[lbndev]

run docker images -q | xargs docker rmi --force before docker system prune -a

[mstenz]

I have the same issue.

docker images -q

or
docker images -f dangling=true

shows 7 images from 8. But these are in use and i don't want to delete them.
@lbndev

TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              8                   8                   3.347GB             933.2MB (27%)
Containers          11                  11                  10.21MB             0B (0%)
Local Volumes       3                   3                   148.3kB             0B (0%)
Build Cache         0                   0                   0B                  0B