Unused docker image disappear occasionally

Hi All,

My docker images (mostly for unused) disappeared for unknown reasons, I already moved my docker storage location to my workspace which is 8T, and the workspace is not full. My configuration of docker is as below:

 Context:    default
 Debug Mode: false
  app: Docker App (Docker Inc., v0.9.1-beta3)
  buildx: Build with BuildKit (Docker Inc., v0.5.1-docker)
  scan: Docker Scan (Docker Inc., v0.8.0)

 Containers: 3
  Running: 3
  Paused: 0
  Stopped: 0
 Images: 2
 Server Version: 20.10.7
 Storage Driver: overlay2
  Backing Filesystem: extfs
  Supports d_type: true
  Native Overlay Diff: true
  userxattr: false
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 1
  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: d71fcd7d8303cbf684402823e425e9dd2e99285d
 runc version: b9ee9c6314599f1b4a7f497e1f1f856fe433d3b7
 init version: de40ad0
 Security Options:
   Profile: default
 Kernel Version: 4.15.0-142-generic
 Operating System: Ubuntu 16.04.7 LTS
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 31.26GiB
 Name: shaochun-gv
 Docker Root Dir: /local/mnt/workspace2/docker-images
 Debug Mode: false

And my workspace:

/dev/sdb1                                             7.3T  5.4T  1.6T  78% /local/mnt/workspace2
overlay                                               7.3T  5.4T  1.6T  78% /local/mnt/workspace2/docker-images/overlay2/0a083dccc4d87253ab39cd7ff6a1048bf0781d7b3d573116943a485bc47d3795/merged
overlay                                               7.3T  5.4T  1.6T  78% /local/mnt/workspace2/docker-images/overlay2/1b10884fc4a7801b1cdcc90187027271d6f8f3f66ae52f22883fbdd602341002/merged
overlay                                               7.3T  5.4T  1.6T  78% /local/mnt/workspace2/docker-images/overlay2/e78c7adf770e886c5b10917af31a06f47b4fdac73872933f9e8f6fb6dd56a333/merged