How to copy /var/lib/docker with overlayfs directory structure with data *as-is* without increasing the storage space

Question

I have a docker installation with several images and about 150Gigs of data in /var/lib/docker. This setup uses overlayfs as its storage driver. There are several directories for each layer under /var/lib/docker/overlay holding the actual data. The partition size is 160G.

My requirement is to copy the the docker directory from /var/lib/docker to a new disk of 1TB, so that I point docker to start from this new partition and continue to use my old images.

Now the problem is, when I use an rsync or a cp command with -a, to copy /var/lib/docker to new partition, instead of a total of 150G actual data, the total copied data is coming to as much as 600G (and counting..).

Docker is stopped as well, but not sure how OS is looking at the 160G data and copying into 600G+. I hope it is not the overlayfs (merged directories). There is no overlay information on df -aTh. Nor did it help unloading kernel overlayfs driver with rmmod overlay

How is it possible that I could just copy this data as-is, without any expansion/merge taking place.

Answer 1

It turned out that docker is using hardlinks within those directories under /var/lib/docker/overlay. Using -H with rsync (copy hardlinks as hardlinks) solved the issue.

rsync -avPHSX /var/lib/docker /new/partition/