Try this:
touch test
chattr +C test
lsattr test
mv test test2
lsattr test2
The original file (test) will have the C flag but when renamed the
flag disappears. If the volume is unmounted and then mounted again the
flag reappears. However, if the file is modified in any way after the
renaming but before the umount/mount then even weirder things happen,
eg:
touch test
chattr +C test
head -c1048576 /dev/zero >> test
lsattr test
# the C flag exists and the file does not COW when modified as expected
mv test test2
lsattr test2
# now the C flag disappeared
# if you modify the file it will COW and the flag will not
reappear after umount/mount
even worse, if you try to use BTRFS_IOC_CLONE to clone test2 to a
non-COW file with eg
cp --reflink test2 test3
it will fail with EINVAL as if the file test2 had the COW flag.
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