[cmurf]

Btrfs internally use logical addresses for all extents. The making from logical to physical is done via the chunk tree which not only indicates physical sector but also the device. So the reference for a file extent says nothing about what device the extent is on or the replication (raid profile) since that is all a function of the chunk and dev trees.

I think Btrfs for a guest F's is best pointed to an LV, rather than qcow2. It's been awhile since I benchmarked that compared to 'qemu-img create -f qcow2 -o nocow=on' which will set xattr +C on the file making it nocow. The nocow xattr helps a lot with this problem.

[ryao]

ZFS also does better when the guests are stored on zvols. If volblocksize=4K is set on creation, it can avoid read-modify-write overhead. Regular files can be used too, but those are somewhat higher overhead. In that case, recordsize=4K can be used.

That said, I do not think the tests involved nesting CoW file systems.