Speed up rsync with Simultaneous/Concurrent File Transfers?

Question

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}/"

Answer 1

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/{}