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On my motherboard I have "SATA 6Gb/s" ports, but when I transfer I typically get 70+ MB/s. It's a "SATA 6Gb/s" disk.

I think I have the same issues with my old PC I use as a server, but it has "SATA 3Gb/s" speed.

What does "SATA 6Gb/s" and "SATA 3Gb/s" mean? Is it total bandwidth, so with 6 SATA ports - working at the same time - I would get 500 MB/s? It's still far off my typical 70+MB/s.

I know the quality of my hard-drive is an issue, it's a plain 3.5 non-SSD, and I know file size and file number make a difference, but is it supposed to get close to 500 MB/s?

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  • You need an SSD to reach anywhere near those numbers all mechanical disks are too slow.
    – cybernard
    Sep 3, 2017 at 22:33

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Sata 6Gbs means the connection can theoretically transfer 6 GigaBits per second BEFORE encoding. This is a pure connection speed and has nothing to do with the capabilities of the device connected.

So on a 6Gb/s link that would be (6,000,000,000 bits/8 bits per byte)*(8/10 link encoding) = 600 MegaBytes per second.

Sata 3Gb/s ends up being 300 MB/s using the same formula.

When you get 70+ MB/s "speed" you are seeing your device is maxing out at 70Mb/s capability and transferring that information over a SATA link capable of 600MB/s.

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  • but 70 mb/s isnt 600 mb/s. Is my disk that bad? Is anyone able to max the 600 mb/s on a single disk? (not doing raid magic)
    – fUrious
    Sep 2, 2017 at 19:37
  • @fUrious To extrapolate, moving 10,000 empty files would show almost 0 MB/s transfer speed on a perfectly healthy drive and could take several minutes if they were located in 10,000 physical locations on the drive!
    – Damon
    Sep 2, 2017 at 19:53
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    @fUrious A HDD has physical limitations. Speed of platters, speed of heads seeking, etc that would limit the speed of your data being transfer into and out of the drive. Read up on how hard drives work. Essentially you can only get it to perform 100-200 physical operations max per second no matter how small the "operation" is. Typically this is the head seeking to proper physical location on the drive.
    – Damon
    Sep 2, 2017 at 19:53
  • @fUrious In general, HDDs cannot max out even a SATA 300 connection without RAID; in general only SSDs are known to be able to saturate any modern SATA bus. Hence PCIe based buses to keep up.
    – Damon
    Sep 2, 2017 at 19:55
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There are two potential questions here:

  1. Why am I getting on 70+ MB/sec?
  2. How does SATA (III) speed corrolate to this.

The last is the easiest. It is the speed that the devices on the SATA cable can communicate with each other. That is 6 giga BIT per second. Actual maximum throughput will be lower due to overhead. 550MB/sec is a reasonably guess and maximum practical thoughput.

As to the first part of the question. The bandwidth is the maximum speed. If a disk is slower then it will not reach these speeds. E.g. a classic rotating harddisk might very well read 70MB/sec from the physical platter. I can transfer that much faster after it is read, but the slowest part oon the chain will limit the speed.

Thus for rotating rust and slow SSDs ther max speed from SATA III is never archieved. With a fast SSD or behind a port mutiplexer it can be reached.

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