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Why size of merged mount folders reflect partition size

Hi all,

Hope everyone well?

We are facing a knowledge issue on an overlay2 point. I precise that we are not using docker registry.

We were upset merged mount point of containers always still full. We were wrong. Merged mount points’s containers reflect state of the partition where is located /var/lib/docker.

If your /var is alone on a partition and this one full because of, guess /var/log, a df -h will display merged mount point as full.

Our question is: why? This is confusing:

Filesystem Size Used Avail Use% Mounted on
/dev/sda3 29G 20G 7.8G 72% /var
overlay 29G 20G 7.8G 72% /var/lib/docker/overlay2/ed4d…/merged
overlay 29G 20G 7.8G 72% /var/lib/docker/overlay2/44d1…/merged
overlay 29G 20G 7.8G 72% /var/lib/docker/overlay2/5455…/merged
overlay 29G 20G 7.8G 72% /var/lib/docker/overlay2/9269…/merged
overlay 29G 20G 7.8G 72% /var/lib/docker/overlay2/e1ec…/merged
root:~#

Is it because we are mounting upperdir and lowerdir and we relay on the filesystem state of these ones?

Cheers & thanks!
Greg