Feature Request for cp(1) utility, consider Btrfs ioctl clone range support

[Jeff Liu]

Hello,

I just found a cool feature in Btrfs for cloning partially source file
to the destination by specifying the desired offset and length to be copied.

The call interface is something like below:
struct btrfs_clone_range_args cr;
cr.src_fd = source_desc;
cr.src_offset = (uint64_t) offset;
cr.src_length = (uint64_t) length;
cr.dest_offset = (uint64_t) d_offset;
ioctl(dest_fd, BTRFS_IOC_CLONE_RANGE, &args);

cp(1) has already works with "reflink" supported for a long time, IMHO,
this new feature could improve it to some extent.
Is it acceptable to add a new option like
"--reflink-range=src_offset,src_length,dst_offset" to clone partially
source file?
In this way, if reflink-range is triggered, cp(1) will try to parse the
offset and length which are separated by comma,
then try to clone src_length bytes from src_offset to the destination
file started at dst_offset.
if reflink-range failed, just let cp(1) fail with appropriate error info.

I'd like to implement it if you guys think it does make sense. :-P.

Thanks,
-Jeff