I have not found any ready-made solution as of today (2016-05-06), but solved the problem for my purposes, including Copy-on-Write handling. The steps to "clone"
Get a list of subvolumes ordered by
btrfs subvolumes list -qu --sort ogen /source. Sorting is probably enough to guarantee that snapshots or subvolumes which depend on previous ones are handled first. This is important for dealing with Copy-on-Write, because we need to have the base volumes transferred first.
Make all subvolumes read-only using
btrfs property set -ts /source/some-volume ro.
Now, for each subvolume from the list above, starting at the top, do the following:
If the volume does not have a parent UUID (displayed as
-) or the parent UUID does not exist anymore in the list, run:
btrfs send /source/some/volume | btrfs receive /target/some/
If the volume does have a parent UUID which still exists, we should have transferred it already because of
--sort ogen and we can use that as a base to avoid data duplication. Hence, find the parent UUID's path in the list and run:
btrfs send -p /source/parent/volume/ -c /source/parent/volume/ /source/some/volume/ | btrfs receive /target/some/ (btrfs would probably guess the
-p argument automatically, but I prefer to be explicit).
After running one of the above commands make the target and source read-write again:
btrfs property set -ts /source/some/volume ro false; btrfs property set -ts /target/some/volume ro false. This step can be skipped if the source has been previously read-only.
This should handle many cases. Caveats:
There might be some complications with respect to ordering when nesting subvolumes/snapshots.
The whole process is obviously more fun when scripted.
btrfs send accepts multiple clone source (
-c) arguments. It may be advantageous to not only specify the parent's volume path, but also those of any ancestors or simply any previously sent volumes. It did not make any difference here, but it might — just a guess — help to avoid data duplication in some cases.
I am unsure if any meta information on snapshots or subvolumes is lost along the way, but just about everything interesting else for most use cases should be preserved.
The whole process helped me transfer an 800 GB filesystem with 3.8 GB used (according to
df) to a 10 GB image with 3.8 GB used. Transferring without
-c would have used about 190 GB, so data duplication was indeed avoided.