GNU parallel with rsync

Are you sure the rsyncs are really not running in parallel ?
Checking with ps | grep rsync while the command is running will show which and how many rsyncs are actually running simultaneously.

By default, parallel holds printing output from each job until it's finished so that the different commands' output don't get all mixed up together:

--group  Group output. Output from each jobs is grouped together and is only printed when the command         is finished. stderr (standard error) first followed by stdout (standard output). This takes         some CPU time. In rare situations GNU parallel takes up lots of CPU time and if it is         acceptable that the outputs from different commands are mixed together, then disabling         grouping with -u can speedup GNU parallel by a factor of 10.         --group is the default. Can be reversed with -u.

My guess is the rsyncs are actually running in parallel, but from the output it feels like they're running serial. -u option changes that.

--

For example with this cmd:

$ for i in 1 2 3 ; do echo a$i ; sleep 1 ; donea1a2a3

By default in parallel we get no feedback until it's all done:

$ (echo a ; echo b ; echo c ) | parallel 'for i in 1 2 3 ; do echo {}$i ; sleep 1 ; done  ' a1a2a3b1b2b3c1c2c3

Whereas with -u stuff get printed right away:

$ (echo a ; echo b ; echo c ) | parallel -u 'for i in 1 2 3 ; do echo {}$i ; sleep 1 ; done  ' a1b1c1a2b2c2a3b3c3

In both cases it took 3s to run though so it's really running simultaneously...