This changeset allows Docker's VFS, and Overlay to take advantage of
Linux's zerocopy APIs.
The copy function first tries to use the ficlone ioctl. Reason being:
- they do not allow partial success (aka short writes)
- clones are expected to be a fast metadata operation
See: http://oss.sgi.com/archives/xfs/2015-12/msg00356.html
If the clone fails, we fall back to copy_file_range, which internally
may fall back to splice, which has an upper limit on the size
of copy it can perform. Given that, we have to loop until the copy
is done.
For a given dirCopy operation, if the clone fails, we will not try
it again during any other file copy. Same is true with copy_file_range.
If all else fails, we fall back to traditional copy.
Signed-off-by: Sargun Dhillon <sargun@sargun.me>
