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Question about docker overlay volume

My understanding of overlay volume is that it's created on-the-fly with CoW. So, if I create a new file in a running container, I expect to see a new volume being created. However, this doesn't seem to be the case:

  1. kaisong@ubuntu-vm:~$ docker volume prune
  2. WARNING! This will remove all local volumes not used by at least one container.
  3. Are you sure you want to continue? [y/N] y
  4. Deleted Volumes:
  5. 32bbf8d5fa65cfdebcb5d5ec6e34007e95bd64825f8119f754c9f22a8c7dc31e
  6. e0e240c78fd0502bf3152b8787646791d90e7e37a93ebda6294aac12dc1d0839
  7. 112f591c70f24aac666e23b61ce37776faa3758497c596a4d43a1fdc0092ca2c
  8. e374aa536721558f33b4ad730ff3dad57d9988d653488cbfadde1ebb409e1e72
  9. 0fdfe34b79dea33d4d25bb6b23cad048ad455fa6d2d31ebabc8c70454223cb6c
  10.  
  11. Total reclaimed space: 3.372GB
  12. kaisong@ubuntu-vm:~$ docker volume ls
  13. DRIVER VOLUME NAME
  14. local cmountvol
  15. kaisong@ubuntu-vm:~$ docker container ls |grep web1
  16. a5882c59e10e nginx "/docker-entrypoint.…" 17 hours ago Up 17 hours 0.0.0.0:80->80/tcp, :::80->80/tcp web1
  17. kaisong@ubuntu-vm:~$ docker container exec -ti web1 sh
  18. # mkdir /data
  19. # touch /data/foobar
  20. # ls /data
  21. foobar
  22. # read escape sequence
  23. kaisong@ubuntu-vm:~$ docker volume ls #Expect a new volume to be created to overlay the "web1" container
  24. DRIVER VOLUME NAME
  25. local cmountvol
  26. kaisong@ubuntu-vm:~$
  27.  

Can anyone help me to understand where the /data/foobar is being saved on the host for the container "web1"?

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