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What is the actual max read/write speed for a SATA III 6.0Gb/s Hard Disk Drive realistically?

I know what 6.0gb/s means, it means six gigabit per second, the giga- prefix uses SI decimal definition, which means 1,000,000,000, and it is 1,000 times a mega- and 1,000,000 times a kilo-, but computers use binary and use Byte as the base unit, 1 Byte = 8 bit, with each unit 1,024 times of the former, so 1 KB is 1,024 Byte and 8,192 bit, 1 MB is 1,048,576 Byte and 8,388,608 bit, and 1 GB is 1,073,741,824 Byte and 8,589,934,592 bit, so 6.0gb/s in decimal would be 732,421.875 KB/s or 715.2557373046875 MB/s in binary.

But in reality, my HDD is Seagate BarraCuda ST1000DM010 1TB 7200 RPM 64MB SATA III 6.0gb/s and connected to a SATA III 6.0gb/s port on my motherboard:

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Yet I have never observed disk read/write speed of this HDD higher than 160MiB/s in taskmgr.exe, using FastCopy with a buffer size of 256MiB to copy a files from the same HDD gives a transfer rate of 128MiB/s at most, and typically around 108MiB/s most of the time, interestingly Get-FileHash can spike the I/O speed of the HDD up to 144MiB/s.

So what is the maximum read/write speed of a SATA III HDD at 7200 rpm with SATA 6.0gb/s interface?

I ask this question because I plan to buy Seagate Exos 7E8 4TB 512n SATA 128MB Cache 3.5-Inch Enterprise Hard Drive (ST4000NM0035), it has 128MB cache instead of mine's 64MB, so what read/write speed should I expect? Would it be about 256MiB/s, because it has cache twice the size of mine, or something higher?

So SATA uses 8b/10b encoding so theoretically the max data rate is 4.8gb/s which equals to 600MB/s or 572.20458984375MiB/s, however this speed is way higher than what I am actually able to get...

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    your observations for Mechanical HDD performance, for a decent 7200RMP disk on SATAIII more or less match mine. if you want to improve the performance substantially you will have to look into SSD. The cache size won't impact your actual speed, just the length of time you can keep at max speed, during sequential R/W. Cache smooths out the bumps, but for continuous usage, it can't even pretend to improve peak read/write speed.
    – Frank Thomas
    Commented Mar 13, 2021 at 8:49
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    "I know what 6.0gb/s means..." -- There's more to it than a simple conversion from a bits to bytes rate. See superuser.com/questions/1432487/data-transfer-rate-query. Also, the SATA interface is but just one sequential operation or transfer in a chain of required operations/transfers to perform a read or write operation. To read or write of a single sector or cluster, these independent operations/transfers must be performed sequentially, one at a time. See superuser.com/questions/350582/…
    – sawdust
    Commented Mar 13, 2021 at 9:38
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    "however this speed is way higher ..." -- The speed of the interface (in this case, SATA) does not have to match the (internal) speed of the storage device (in this case, a HDD). Both commands and data are transmitted over SATA. You want a fast interface. It's like a trip with one leg is by car and the next leg is by plane. You're complaining that the car is too slow and the plane is too fast. The independent but sequential phases do not require speeds to match.
    – sawdust
    Commented Mar 13, 2021 at 20:00

1 Answer 1

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The SATA interface runs at 6 Gbps, which due to the 8-bit bytes being encoded into 10-bit symbols with 8b10b line code, equals to 600 MB/s.

It is just that a mechanical rotating hard drive cannot achieve such high read or write speeds.

It even reads in the Seagate ST1000DM010 manual what kind of burst and sustained read/write speeds can be achieved. The average is 156 MB/s even if the interface speed is 600 MB/s.

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  • Stop editing the post with false information. SATA 6Gbps link is NOT able to transfer 750 MB/s, it is 600 MB/s due to 8-bit bytes being encoded to 10-bit symbols.
    – Justme
    Commented Feb 13, 2023 at 6:35
  • the crystaldisk mark on an SSD sata 6 pegged at 766.67 Mb/s read 789.77 Mb/s write shows it is much closer to 750MB/s than 600.
    – Rostol
    Commented Oct 20, 2023 at 14:24
  • @Rostol Then it means your test program is wrong. It is physically impossible to transfer that amount of data through 8b10b encoded 6 Gbps link as it only leaves you 4.8 Gbps of uncoded data rate which is 600 MB/s. You can verify that yourself by reading Wikipedia article on SATA or the SATA specifications themselves, instead of blindly believing results of a test program.
    – Justme
    Commented Oct 20, 2023 at 14:44
  • @Justme "Blindly trusting" is a bit harsh when there are real-world results that seem to contradict your statements. Of course, you are correct about the technical limitations of SATA 3.0 (ie, SATA 6 gbps), but that doesn't mean Rostol didn't have valid results with other unexplained parameters. Explanations could include using a different protocol than SATA and RAID configurations.
    – Zediiiii
    Commented Nov 7, 2023 at 15:34
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    @justme. Those are definitely possible explanations, probably even the most likely explanations. I agree with you, but I also wanted to point out that there are multiple possible explanations, and not all of them are that "human was falling into the trap of blatantly inflating the limits of SATA 3 theoretical speeds." Your tone comes off aggressive and harsh. To a reseller or manufacturer rep selling storage, that's more warranted. To a human trying to learn, it's rude and unnecessary. You obviously know your stuff. Respectfully, more people will listen to you if you use a friendlier tone.
    – Zediiiii
    Commented Nov 8, 2023 at 16:49

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