I’m currently watching the progress of a 4tB rsync file transfer, and i’m curious why the speeds are less than the theoretical read/write maximum speeds of the drives involved with the transfer. I know there’s a lot that can effect transfer speeds, so I guess i’m not asking why my transfer itself isn’t going faster. I’m more just curious what the bottlenecks could be typically?

Assuming a file transfer between 2 physical drives, and:

  • Both drives are internal SATA III drives with 5.0GB/s 5.0Gb/s read/write 210Mb/s (this was the mistake: I was reading the sata III protocol speed as the disk speed)
  • files are being transferred using a simple rsync command
  • there are no other processes running

What would be the likely bottlenecks? Could the motherboard/processor likely limit the speed? The available memory? Or the file structure of the files themselves (whether they are fragmented on the volumes or not)?

  • @[email protected]English
    3•11 months ago

    As I mentioned in my previous post below, even in theory a spinning platter is not going to reach anywhere near 5Gb/s speeds, not even 1/20th of that. You can google the specs as easily as I can, but a 4TB WD Blue drive is only 5400rpm which seriously hampers its speed, limiting it to about 175MB/s (bytes, not bits).

    The 4TB Seagate Ironwolf is another slow drive at only 5900rpm, but does manage to creep up to about 190MB/s transfer speeds.

    You didn’t mention which one is your 4TB drive, but the speed of the slowest drive is going to dictate your top transfer speeds. No matter how you slice it, you can expect a long wait to transfer 4TB of data. If you want more speed, you can get better performing 7200rpm drives, but you won’t see any substantial increases until you move into a multi-drive RAID. I would recommend a minimum of 5 drives, but for comparison I have eight 18TB drives set up through ZFS as a raidZ2 configuration (similar to RAID6) which gives me a sustained transfer rate of around 450MB/s. If you need faster, you really have no choice but to upgrade to SSD.

    • archomrade [he/him]OPEnglish
      3•11 months ago

      Thank you for this, I have clearly misunderstood the rated speeds of the drives.

      Now I feel silly for having thought the 6Gb/s stated in the product title on Microcenter as an indication of the speed (and for having not thought twice about it). It does say in the product details: “[…] data transfer speeds of up to 210 MB/s”. I guess they were simply saying that 6Gb/s is what the SATAIII interface is capable of? 6Gb/s is in the listed product title on amazon/micro center, and I was obviously duped by this.

      I feel a little silly having believed it without really questioning it.

      • @[email protected]English
        2•11 months ago

        Haha and now you know exactly WHY they do that! The manufacturers were more than happy to let people keep believing SATA3 drives would be faster than SATA2 drives until they started facing public backlash and the costs of returns, but they still try to bury it in the fine print.

        Keep in mind that any transfer speeds on the box are also going to be best-case scenarios, for read access only (because writing takes longer than reading even on an SSD). The numbers I found on reviews are generally going to be more real-world conditions including a combination of simultaneous read/write operations. Personally I don’t trust anything except what I can get in my own installations because everyone’s hardware and software are different, but if you decide to do your own testing make sure that it disables cached operations during the tests or you’re not doing anything but checking the speed of your RAM.