When copying to the local backend with checksums enabled (the default),
rclone hashed the incoming data by wrapping the source reader in an
io.TeeReader. TeeReader has no WriteTo method, so io.Copy could not use
the source's fast path and fell back to its generic 32 KiB buffer loop.
The same wrapping also stopped the destination *os.File using
copy_file_range, since the source was no longer a raw fd.
This meant checksummed copies were written in 32 KiB chunks whereas
--ignore-checksum copies were written in much larger blocks (typically
1 MiB). On filesystems where small writes are expensive, such as FUSE
mounts like LucidLink, this made a big difference: copying a 100 MiB
file took 3203 x 32 KiB writes in 3.6s, and now takes 108 x ~1 MiB
writes in 0.47s.
Dominik Sander
e2352201d1