I used this code to split a big file into chunks and upload it to Google Cloud Storage . The use case is "How to upload a huge file to GCS in parts so you can overcome the 5 Tb limits in GCS and then join it back with cat or gcloud cat.

splitter.go

package main

import (
	"bufio"
	"cloud.google.com/go/storage"
	"context"
	"flag"
	"fmt"
	"os"
	"strings"
)

/**
 * I used this code to split a big file into chunks and upload it to Google Cloud Storage
 * The use case is "How to upload a huge file to GCS in parts so you can overcome the 5 Tb limits in GCS
 * and then join it back with cat or gcloud cat.
 * @Author: iolalla@gmail.com
 */

var filez string
var chunkSize int
var destination string

func main() {

	flag.StringVar(&filez, "file", "tradeinn.tar.gz", "File to split")
	flag.IntVar(&chunkSize, "size", 500000, "Size in Byte of the parts")
	flag.StringVar(&destination, "dest", ".", "Where to write the file, if it starts with gs, will save it in Google Cloud Storage")
	flag.Parse()

	file, err := os.Open(filez)
	if err != nil {
		panic(err)
	}
	defer file.Close()

	// to divide file in chunks of chunkSize in Bytes
	info, _ := file.Stat()
	chunks := int(info.Size()) / chunkSize

	// reader of chunk size
	bufR := bufio.NewReaderSize(file, chunkSize)

	// We need to add a final slice for the final bytes
	var slice = make([]int, chunks+1)

	if strings.HasPrefix(destination, "gs://") {
		for i := range slice {
			reader := make([]byte, chunkSize)
			rlen, err := bufR.Read(reader)
			// fmt.Println("Read: ", rlen)
			if err != nil {
				panic(err)
			}
			writeGCS(i, rlen, &reader)
		}
	} else {
		for i := range slice {
			reader := make([]byte, chunkSize)
			rlen, err := bufR.Read(reader)
			if err != nil {
				panic(err)
			}
			writeFile(i, rlen, &reader)
		}
	}
}

func writeGCS(i int, rlen int, bufW *[]byte) {
	ctx := context.Background()
	client, err := storage.NewClient(ctx)
	if err != nil {
		panic(err)
	}
	fname := fmt.Sprintf("%v/file_%v", filez, i)
	path := strings.ReplaceAll(destination, "gs://", "")
	wc := client.Bucket(path).Object(fname).NewWriter(ctx)
	wbytes := *(bufW)
	if _, err := wc.Write(wbytes[:rlen]); err != nil {
		panic(err)
	}
	if err := wc.Close(); err != nil {
		panic(err)
	}
	fmt.Println("gcloud cat ", destination, "/", fname, " >> ", filez)
}

func writeFile(i int, rlen int, bufW *[]byte) {
	fname := fmt.Sprintf("file_%v", i)
	f, err := os.Create(fname)
	defer f.Close()

	w := bufio.NewWriterSize(f, rlen)
	wbytes := *(bufW)
	_, err = w.Write(wbytes[:rlen])
	if err != nil {
		panic(err)
	}
	fmt.Println("cat ", fname, " >> ", filez)
	w.Flush()
}