Motivation

SSH is often used to transfer files (or streams), by scp (always) and often by rsync (if used over ssh, i.e. with option --rsh ssh).

By default ssh as well as rsync tend to try to compress the data transferred. This is useful over slow connections, like a typical SOHO internet line, especially when using the slow upload channel of asynchronous technologies or plans.

If the network connection is fast, as in a GBit connection within a LAN or DMZ, even with top-notch CPUs those compressions become bottlenecks and reduce the transferspeed from say 100 MB/s to 25 MB/s or even less.

Therefore it sometimes makes a lot of sense and saves a lot of time to suppress those compression attempts.

Supressing ssh's compression

To suppress ssh's compression for a specific scp call, add -o 'Compression no'.

To generally suppress ssh's compression for connections to certain hosts create or adapt Host section(s) in /etc/ssh/ssh_config or ~/.ssh/config, like this:

Host *.dmz.foobar
Compression no

This applies to all ssh conctions to those host(s), wether you use ssh, scp, or rsync over ssh) 

Suppressing rsync's compression

If calling rsync directly, do NOT use --compress (resp. -z).

Some rsync wrappers like our xsync script add (i.e. "inject") --compress for remote connections automatically, but allow to pass through further rsync options. In that case add --compress-level=0.

Special note for rsync over ssh

If using rsync over ssh, both tools's compression attempts must be avoided seperately!