Much improved speeds of rsync via SSH - something to consider

Karl O. Pinc kop at
Mon Apr 2 13:51:34 UTC 2018

On Mon, 2 Apr 2018 15:05:14 +0200
Thanassis Tsiodras via rsync <rsync at> wrote:

> I recently concluded a bug hunt to trace why my rsync-ing to an SBC
> was much slower than the corresponding iperf3-reported speeds. To
> give a concise summary of the situation, in slow wifi links using SSH
> with ProxyCommand tremendously speeds up things:

>     $ rsync -avz --progress -e 'ssh -o "ProxyCommand nc %h %p"'
> ./ root at

> If it isn't clear - the speed of the upload went from 543 kbytes/sec
> to 2690 kbytes/sec.
> If you want to see why - and how I traced this down - you can read my
> complete report in the UNIX StackExchange forum. To avoid being
> classified as a spammer, I won't include a link - search for question
> 434825 in the search bar. The executive summary is that SSH disables
> Nagle's algorithm by default - and in slow links this can cause
> tremendous impact (as you see above).
> Hope this helps people that backup over slow links (wi-fi or
> otherwise).

In the interest of collecting ssh-related issues in a single
thread I add this note:

The HPN patches to the portable OpenSSH implementation can
improve speed over high speed links with long round trip times.

It also helps in those cases where CPU is a bottleneck and
encryption of transmitted data unnecessary.

The FAQ is also interesting.

The executive summary is:

  SCP and the underlying SSH2 protocol implementation in OpenSSH is
  network performance limited by statically defined internal flow
  control buffers.


Karl <kop at>
Free Software:  "You don't pay back, you pay forward."
                 -- Robert A. Heinlein

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