About rsync over SSH and compression

JCA 1.41421 at gmail.com
Wed Jun 26 02:46:25 AEST 2019

Thanks; I did not think of that. I have just run a few basic tests with
both rsync and OpenSSH in their default settings, when it comes to
compression. SSH compression seems to have a very slight edge. However, I
surmise that, given the number of knobs available on both sides (OpenSSH,
in particular) one can tinker with settings almost endlessly in either
side, probably being able to end up with specific situations where one or
the other would be advantageous. At first sight, however, the overall
difference seems to be small.

On Tue, Jun 25, 2019 at 10:33 AM Tim Broberg <Tim.Broberg at servicenow.com>

> Those are all good points.
> A few more to consider:
> 1 - You may want to exert more effort compressing your rsync traffic than
> you would want to apply across the board in ssh.
> 2 - You *know* rsync supports compression on both sides. You don't
> necessarily control what algorithms are turned on on both ends in ssh.
> I think I would lean towards doing it in the application myself.
>     - Tim.
> On 6/25/19, 9:11 AM, "openssh-unix-dev on behalf of JCA"
> <openssh-unix-dev-bounces+tim.broberg=servicenow.com at mindrot.org on
> behalf of 1.41421 at gmail.com> wrote:
>     [External Email]
>     Rsync supports the capability of compressing data before sending. So
> does
>     OpenSSH. It would be probably be a waste of resources and time to
> enable
>     both compression capabilities at the same time, but it is not clear to
> me
>     whether, in general, it makes better sense to enable rsync compression
> or
>     SSH compression.
>     My first thought would be that SSH compression might yield better
> results,
>     on the ground that SSH will try to cram as much data as possible in a
>     channel data message, within the limitations imposed by the maximum SSH
>     message size and the current window size. On the other hand, rsync
> might
>     (and 'might' is the keyword here) resort to individual
>     SSH2_MSG_CHANNEL_DATA messages for particular deltas, thus giving the
>     compression code smaller amounts of data to play with every time.
>     Additionally, SSH compression will be able to compress the rsync
> protocol
>     control data, which rsync will not be able to compress (right?)
>     Feedback on the issues above will be much appreciated.
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