`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