The Btrfs filesystem uses the terms
default subvolume somewhat confusingly to me.
An example is the btrfs wiki page about subvolumes which states
A freshly created filesystem is also a subvolume, called top-level, internally has an id 5. This subvolume cannot be removed or replaced by another subvolume.
However, as using tools like
btrfs-progs commands such as
- btrfs subvolume list
- btrfs subvolume show
exhibit, the term subvolume does not apply really to the rootvolume (see this other question). Additionally the term
top-level (as can be seen in the output of the above commands, is not really limited to the subvolume/root volume with subvolumeid 5. Finally it seems you cannot ever
btrfs send the root volume, just subvolumes (again contrasting to the definitions above).