Speed up rsync with Simultaneous/Concurrent File Transfers?

We need to transfer 15TB of data from one server to another as fast as we can. We're currently using rsync but we're only getting speeds of around 150Mb/s, when our network is capable of 900+Mb/s (tested with iperf). I've done tests of the disks, network, etc and figured it's just that rsync is only transferring one file at a time which is causing the slowdown.

I found a script to run a different rsync for each folder in a directory tree (allowing you to limit to x number), but I can't get it working, it still just runs one rsync at a time.

I found the script here (copied below).

Our directory tree is like this:

/main
   - /files
      - /1
         - 343
            - 123.wav
            - 76.wav
         - 772
            - 122.wav
         - 55
            - 555.wav
            - 324.wav
            - 1209.wav
         - 43
            - 999.wav
            - 111.wav
            - 222.wav
      - /2
         - 346
            - 9993.wav
         - 4242
            - 827.wav
      - /3
         - 2545
            - 76.wav
            - 199.wav
            - 183.wav
         - 23
            - 33.wav
            - 876.wav
         - 4256
            - 998.wav
            - 1665.wav
            - 332.wav
            - 112.wav
            - 5584.wav

So what I'd like to happen is to create an rsync for each of the directories in /main/files, up to a maximum of, say, 5 at a time. So in this case, 3 rsyncs would run, for /main/files/1, /main/files/2 and /main/files/3.

I tried with it like this, but it just runs 1 rsync at a time for the /main/files/2 folder:

#!/bin/bash

# Define source, target, maxdepth and cd to source
source="/main/files"
target="/main/filesTest"
depth=1
cd "${source}"

# Set the maximum number of concurrent rsync threads
maxthreads=5
# How long to wait before checking the number of rsync threads again
sleeptime=5

# Find all folders in the source directory within the maxdepth level
find . -maxdepth ${depth} -type d | while read dir
do
    # Make sure to ignore the parent folder
    if [ `echo "${dir}" | awk -F'/' '{print NF}'` -gt ${depth} ]
    then
        # Strip leading dot slash
        subfolder=$(echo "${dir}" | sed 's@^\./@@g')
        if [ ! -d "${target}/${subfolder}" ]
        then
            # Create destination folder and set ownership and permissions to match source
            mkdir -p "${target}/${subfolder}"
            chown --reference="${source}/${subfolder}" "${target}/${subfolder}"
            chmod --reference="${source}/${subfolder}" "${target}/${subfolder}"
        fi
        # Make sure the number of rsync threads running is below the threshold
        while [ `ps -ef | grep -c [r]sync` -gt ${maxthreads} ]
        do
            echo "Sleeping ${sleeptime} seconds"
            sleep ${sleeptime}
        done
        # Run rsync in background for the current subfolder and move one to the next one
        nohup rsync -a "${source}/${subfolder}/" "${target}/${subfolder}/" </dev/null >/dev/null 2>&1 &
    fi
done

# Find all files above the maxdepth level and rsync them as well
find . -maxdepth ${depth} -type f -print0 | rsync -a --files-from=- --from0 ./ "${target}/"

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/{}

rsync transfers files as fast as it can over the network. For example, try using it to copy one large file that doesn't exist at all on the destination. That speed is the maximum speed rsync can transfer data. Compare it with the speed of scp (for example). rsync is even slower at raw transfer when the destination file exists, because both sides have to have a two-way chat about what parts of the file are changed, but pays for itself by identifying data that doesn't need to be transferred.

A simpler way to run rsync in parallel would be to use parallel. The command below would run up to 5 rsyncs in parallel, each one copying one directory. Be aware that the bottleneck might not be your network, but the speed of your CPUs and disks, and running things in parallel just makes them all slower, not faster.

run_rsync() {
    # e.g. copies /main/files/blah to /main/filesTest/blah
    rsync -av "$1" "/main/filesTest/${1#/main/files/}"
}
export -f run_rsync
parallel -j5 run_rsync ::: /main/files/*

You can use xargs which supports running many processes at a time. For your case it will be:

ls -1 /main/files | xargs -I {} -P 5 -n 1 rsync -avh /main/files/{} /main/filesTest/