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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?

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4  
What you are doing does not make any sense. The purpose of rsync is to speed up transferring files over the network, not locally. In order to find the differences, it has to read both the source and destination. In the time it takes to read the destination locally to find the differences, you may as well just do a normal copy. Just download the file to the destination in the first place instead of copying it. –  psusi Jan 17 '11 at 15:35
1  
So it just does not use delta-xfer because, working locally, it's faster to copy than to calculate hashes? If yes — post the answer plz :) –  kolypto Jan 17 '11 at 16:37
3  
Reading can be faster than writing to a local disk in certain circumstances. It can also reduce the wear on an SSD. This is certainly a valid question and the answer is quite valuable to me. –  HRJ May 8 '13 at 11:56
    
@psusi apart from HRJ's comment above, also consider the case when the target file has been reflinked (eg on btrfs or ocfs2). Minimizing writes during the sync can make an enormous difference to the overall space usage. –  Jack Douglas Aug 23 '13 at 21:13

2 Answers 2

up vote 9 down vote accepted

According to the manpage psusi is right:

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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7  
Oh, thank you! I miseed this line :) To turn delta-trasfer on, use -no-W –  kolypto Jan 18 '11 at 12:35

The straight-forward answer to this question is:

Use the --no-W flag to force delta compression, no matter local or remote.

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