btrfs and cp

Chris Mason wrote to linux-btrfs:

"[..] Looks like the latest versions of cp use fiemap to decide if a file is

sparse, which is a great way to avoid doing memcmp, but only if your
fiemap is really accurate about which ranges in the file have holes.

Our fiemap tends to return holes for delalloc bytes, and has a few other
bugs along the way.  The end result is a bunch of zeros instead of good
data when you copy the file. [..]"

patch is available here