@lamp watchSync() {
watch -n1 'grep -E "(Dirty|Write)" /proc/meminfo; echo; ls /sys/block/ | while read device; do awk "{ print \"$device: \" \$9 }" "/sys/block/$device/stat"; done'
}

GNU sync will exit when writeback is complete, although it won't tell you how much there is to flush unlike watchSync.