如题，多线程分段下载文件时，为什么不下载到同一个大文件中？而是要分别下载到单独的文件然后再合并。我自己写了个 demo ，假设开 50 个线程下载，每个线程下载时间为 10s 钟，可以看到最后写入文件的时间才 1s 不到。所以下载文件时，瓶颈是网络 IO 吧，操作文件 IO 的时间可以忽略不计了。所以是不是下载到一个文件中更优？

我的Demo代码如下：

Cargo.toml

[package]
name = "reqwest_download"
version = "0.1.0"
edition = "2021"

[dependencies]
tokio = { version = "~1.34.0", features = ["full"] } # Event-driven, non-blocking I/O platform.
anyhow="1.0"
reqwest = { version = "0.12", features = ["stream"] }
futures = "0.3"
openssl = { version = "0.10", features = ["vendored"] }
futures-util = "0.3.30"

use std::fmt::format;
use std::time::Duration;

use anyhow::ensure;
use futures_util::StreamExt;
use reqwest::Client;
use tokio::fs::File;
use tokio::io::SeekFrom;
use tokio::io::{self, AsyncReadExt, AsyncSeekExt, AsyncWriteExt};
use tokio::task::JoinSet;
use tokio::time::{sleep, Instant};
#[tokio::main]
async fn main() -> Result<(), anyhow::Error> {
    let now = Instant::now();
    test1().await;
    println!("{}ms", now.elapsed().as_millis());
    Ok(())
}
async fn test1() -> Result<(), anyhow::Error> {
    let mut set = JoinSet::new();
    let byte_array: Vec<u8> = (0..=1024 * 1024).map(|x| x as u8).collect();

    for start in 0..50 {
        let cloned_bytes = byte_array.clone();
        set.spawn(async move {
            download_file1(start, cloned_bytes).await;
        });
    }
    while let Some(res) = set.join_next().await {
        let out = res?;
    }
    Ok(())
}
async fn test2() -> Result<(), anyhow::Error> {
    let mut set = JoinSet::new();
    let byte_array: Vec<u8> = (0..=1024 * 1024).map(|x| x as u8).collect();

    for start in 0..50 {
        let cloned_bytes = byte_array.clone();
        set.spawn(async move {
            download_file2(start, cloned_bytes).await;
        });
    }
    while let Some(res) = set.join_next().await {
        let out = res?;
    }
    Ok(())
}
async fn download_file1(start: i32, byte_array: Vec<u8>) -> Result<(), anyhow::Error> {
    let mut file = tokio::fs::OpenOptions::new()
        .create(true)
        .write(true)
        .open("test.bin")
        .await?;
    let current = start * byte_array.len() as i32;
    sleep(Duration::from_secs(10)).await;
    file.seek(SeekFrom::Start(current as u64)).await;
    file.write_all(&byte_array).await;
    Ok(())
}
async fn download_file2(start: i32, byte_array: Vec<u8>) -> Result<(), anyhow::Error> {
    let file_name = format!("{}.bin", start);
    let mut file = tokio::fs::OpenOptions::new()
        .create(true)
        .write(true)
        .open(file_name)
        .await?;
    let current = start * byte_array.len() as i32;
    sleep(Duration::from_secs(10)).await;
    file.write_all(&byte_array).await;
    Ok(())
}