Slow large file copy

Hi all,
rsync seems slow to copy large files. The issue is not related to the 
transfer itself, which is quite fast, but to the discovery of different 
blocks.

For example, transferring a big (multi-GB) file from src (remote) to dst 
(local) with "--inplace" (to avoid a whole-file copy on dst) shows the 
following:

- if dst does not have a local file, or if -W was given, the copy 
immediately starts (but it clearly needs to transfer all blocks);

- if dst has a previous file version (and -W was not given), first dst 
reads the entire file, then src does the same, next different blocks are 
copied and finally dst re-read the entire file.

As dst and src file scans do not happen concurrently, and including the 
last whole-file read by dst, copy time is vastly increased at about 3x 
compared to an optimized process (with concurrent scans and without the 
final read).

Am I missing something? Can rsync be faster for large-file copies?

Regards.

-- 
Danti Gionatan

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