Parallel-ise an rsync transfer when you want multiple concurrent transfers happening,

rsync_parallel.sh

#!/bin/bash
set -e

# Usage:
#   rsync_parallel.sh [--parallel=N] [rsync args...]
# 
# Options:
#   --parallel=N	Use N parallel processes for transfer. Defaults to 10.
#
# Notes:
#   * Requires GNU Parallel
#   * Use with ssh-keys. Lots of password prompts will get very annoying.
#   * Does an itemize-changes first, then chunks the resulting file list and launches N parallel
#     rsyncs to transfer a chunk each.
#   * be a little careful with the options you pass through to rsync. Normal ones will work, you 
#     might want to test weird options upfront.
#

if [[ "$1" == --parallel=* ]]; then
	PARALLEL="${1##*=}"
	shift
else
	PARALLEL=10
fi
echo "Using up to $PARALLEL processes for transfer..."

TMPDIR=$(mktemp -d)
trap "rm -rf $TMPDIR" EXIT

echo "Figuring out file list..."
# sorted by size (descending)
rsync $@ --out-format="%l %n" --no-v --dry-run | sort -n -r > $TMPDIR/files.all

# check for nothing-to-do
TOTAL_FILES=$(cat $TMPDIR/files.all | wc -l)
if [ "$TOTAL_FILES" -eq "0" ]; then
	echo "Nothing to transfer :)"
	exit 0
fi

function array_min {
	# return the (index, value) of the minimum element in the array
	IC=($(tr ' ' '\n' <<<$@ | cat -n | sort -k2,2nr | tail -n1))
	echo $((${IC[0]} - 1)) ${IC[1]}
}

echo "Calculating chunks..."
# declare chunk-size array
for ((I = 0 ; I < PARALLEL ; I++ )); do
	CHUNKS["$I"]=0 
done

# add each file to the emptiest chunk, so they're as balanced by size as possible
while read FSIZE FPATH; do
	MIN=($(array_min ${CHUNKS[@]}))
	CHUNKS["${MIN[0]}"]=$((${CHUNKS["${MIN[0]}"]} + $FSIZE))
	echo $FPATH >> $TMPDIR/chunk.${MIN[0]}
done < $TMPDIR/files.all

find "$TMPDIR" -type f -name "chunk.*" -printf "\n*** %p ***\n" -exec cat {} \;

echo "Starting transfers..."
find "$TMPDIR" -type f -name "chunk.*" | parallel -j $PARALLEL -t --verbose --progress rsync --files-from={} $@

I updated my zsh reimplementation of this script so it should now handle filenames with any funny characters in them. It still starts transferring immediately, without waiting for the full file list to be built first. Comments welcome.

Amazing work @akorn!