Speed up rsync with Simultaneous/Concurrent File Transfers?

 

Updated answer (Jan 2020)

xargs is now the recommended tool to achieve parallel execution. It's pre-installed almost everywhere. For running multiple rsync tasks the command would be:

ls /srv/mail | xargs -n1 -P4 -I% rsync -Pa % myserver.com:/srv/mail/

This will list all folders in /srv/mail, pipe them to xargs, which will read them one-by-one and and run 4 rsync processes at a time. The % char replaces the input argument for each command call.

Original answer using parallel:

ls /srv/mail | parallel -v -j8 rsync -raz --progress {} myserver.com:/srv/mail/{}

apt-get install moreutils to install parallel

apt-get install parallel