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Question about docker overlay volume

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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:

kaisong@ubuntu-vm:~$ docker volume prune
WARNING! This will remove all local volumes not used by at least one container.
Are you sure you want to continue? [y/N] y
Deleted Volumes:
32bbf8d5fa65cfdebcb5d5ec6e34007e95bd64825f8119f754c9f22a8c7dc31e
e0e240c78fd0502bf3152b8787646791d90e7e37a93ebda6294aac12dc1d0839
112f591c70f24aac666e23b61ce37776faa3758497c596a4d43a1fdc0092ca2c
e374aa536721558f33b4ad730ff3dad57d9988d653488cbfadde1ebb409e1e72
0fdfe34b79dea33d4d25bb6b23cad048ad455fa6d2d31ebabc8c70454223cb6c

Total reclaimed space: 3.372GB
kaisong@ubuntu-vm:~$ docker volume ls
DRIVER    VOLUME NAME
local     cmountvol
kaisong@ubuntu-vm:~$ docker container ls |grep web1
a5882c59e10e   nginx     "/docker-entrypoint.…"   17 hours ago     Up 17 hours     0.0.0.0:80->80/tcp, :::80->80/tcp   web1
kaisong@ubuntu-vm:~$ docker container exec -ti web1 sh
# mkdir  /data
# touch /data/foobar
# ls /data      
foobar
# read escape sequence
kaisong@ubuntu-vm:~$ docker volume ls #Expect a new volume to be created to overlay the "web1" container
DRIVER    VOLUME NAME
local     cmountvol
kaisong@ubuntu-vm:~$

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

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