If available FICLONE and copy_file_range will be tried in addition to sendfile
when copying data between two files. The fallback is a tradition read/write
loop. On systems that support these it should improve performance.
When enabled if a regular file is opened which has a link count > 1 it will
copy the file to a temporary file and rename over the original. Effectively
breaking the link. This behavior is similar to cow-shell and other LD_PRELOAD
based "CoW" solutions.
This feature mimics the standard mhddfs behavior but is more thorough.
If a write fails and the errno is set to ENOSPC then mergerfs will (if
the feature is enabled) attempt to move the file to the drive with
the most free space but only if it has enough room for the file plus
the amount to be written. If that transfer is successful it will then
unlink the original file and attempt the previously failed write again.
The copy includes copying the path and file including the acls, owners,
attributes, extended attributes, and timestamps.
