Linux – Why does’t rsync use delta-transfer for local files

linuxrsync

I have a big iso image which is currently being downloaded by a torrent client with space-reservation turned on: that means, file size is not changing while some chunks in in (4 Mib) are constantly changing because of a download.

At 90% download I do the initial rsync to save time later:

$ rsync -Ph DVD.iso /media/another-hdd/
sending incremental file list

DVD.iso
       2.60G 100%   40.23MB/s    0:01:01 (xfer#1, to-check=0/1)

sent 2.60G  bytes  received 73 bytes  34.59M bytes/sec
total size is 2.60G   speedup is 1.00

Then, when the file's fully downloaded, I rsync again:

total size is 2.60G   speedup is 1.00

Speedup=1 says delta-transfer was not used, although 90% of the file has not changed, target dir is on another FS and copying takes several minutes. Why doen't it try to speedup the transfer?! How can I force rsync to use delta-transfer?

Best Answer

According to the manpage, psusi is right:

-W, --whole-file: The transfer may be faster if this option is used when the bandwidth between the source and destination machines is higher than the bandwidth to disk (especially when the "disk" is actually a networked filesystem). This is the default when both the source and destination are specified as local paths, but only if no batch-writing option is in effect.

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