pack huge directory(billion files) to multiple tar files

pack.sh

#!/bin/bash

export PATH="/usr/local/bin:/usr/bin:/bin:/usr/local/sbin:/usr/sbin:/sbin"
export PACK_ROOT="${PACK_ROOT:-/}"
export PACK_NAME="${PACK_NAME:-$(shuf -zer -n5 {a..z})}"
export PACK_SIZE="${PACK_SIZE:-10000}"
export PACK_JOBS="${PACK_JOBS:-16}"

while [[ "$#" -gt 1 ]]; do
    case $1 in
        -C|--root) PACK_ROOT="$2"; shift ;;
        -n|--name) PACK_NAME="$2"; shift ;;
        -s|--size) PACK_SIZE="$2"; shift ;;
        -j|--jobs) PACK_JOBS="$2"; shift ;;
        *) echo "Unknown parameter passed: $1"; exit 1 ;;
    esac
    shift
done

# --exclude-tag,,, https://stackoverflow.com/a/13296077/1355228
tar -c -v -P -f /dev/null --exclude-tag ".void" -C "$PACK_ROOT" "$1" \
    | tee "${PACK_NAME}.index" \
    | split -a 5 -d -l $PACK_SIZE --additional-suffix=".tmp" - "$PACK_NAME."

for one in $(ls -1 ${PACK_NAME}.*.tmp); do
    num="${one%.tmp}"    # some.thing.01234.tmp  ->  some.thing.01234
    num="${num##*.}"     # some.thing.01234      ->  01234
    [[ $num =~ ^-?[0-9]+$ ]] || continue

    # split thousands file into multiple sub-folders
    #   00034,00134,01234,01034,10134,...,99934  ->  34
    dir="$(printf %02d $(expr $num % 100))"
    mkdir -p $dir

    log="$dir/${one%%.tmp}.log"
    tar="$dir/${one%%.tmp}.tar.zst"
    [[ -s $one ]] || continue
    [[ -s $tar ]] && continue

    sem -j $PACK_JOBS "tar \
        -c --zstd --numeric-owner --no-recursion \
        -P -V "$PACK_ROOT" -C "$PACK_ROOT" \
        -vv --index-file=$log -T $one -f $tar \
        && rm -f -- $one"
#   sem -j $PACK_JOBS "dar -Q -zlzop-3 -R $1 --include-from-file $one -c $PACK_NAME"
done
sem --wait