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Disk test doesn't max NVME speeds.

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  • Disk test doesn't max NVME speeds.

    Why doesn't the PerformanceTest drive benchmark test nvme drives at top speed? Running the benchmark on 2 980pro's separately and combined in a raid0 array the test never goes over 4,343MB/s. The top speed on 1 - 980pro is over 7,000MB/s and 2 in a raid0 have a top speed over 11,000. I've noticed this since the 980pro came out and I've been meaning to post this question for a while.

  • #2
    I think you'll find the disks are 100% busy during the test run. Meaning they can't go any faster in your system under this test scenario. The vendors of these drives like to claim huge speeds, but in real life scenarios they almost never hit the claimed speeds.

    Try running some of the advanced tests in PerformanceTest instead.

    (if they aren't 100% busy then maybe there is some other bottleneck in your system. e.g. your RAID controller or the CPU, or the PCIe bus).


    • #3
      No the disks are not 100% busy in my system during the tests. I will take screenshots to show what the top speed is and the disk utilization during passmark testing and show what I am talking about. I've been noticing this for a while and I assure you that the disk test isn't running my disks at full speed.


      • #4
        You should see something like this (screen shot below) for the standard disk read test.

        Note the disk "Active time" is 100%.
        This is for a Western Digital BLACK SN850 NVMe SSD (PCIe 4, 500GB)
        CPU: Ryzen 5 5600X
        RAM: 32GB DRR4

        Western Digital claim this drive does 7000MB/sec reads, but this largely marketing bullshit. Here you can see the drive is 100% busy reading, but only doing 4.2GB/Sec.

        But also note that there is some CPU load. At these high speeds it is relatively easy that a slow CPU or PCIe bus (especially if slow in single threading) can be a bottle neck. Also note that there is 106KB/sec in writing going on. This isn't from the benchmark, but is background activity from Windows (it is a system drive being benchmarked in this case). Too much of this and it can also have a negative effect on the benchmark.

        I'll do a 2nd post with some details from the Advanced tests in PerformanceTest.

        Click image for larger version  Name:	DiskReadMaxLoad.png Views:	0 Size:	251.1 KB ID:	50587


        • #5
          Below is the "Drive Performance" disk benchmark test under the Advanced Test menu in PerformanceTest.

          It runs a bunch of disk test scenarios that are reasonably accepted in the industry as reasonable tests to run.

          This is for the same WD drive as above (that WD's marketing people claim is a 7000MB/sec drive)

          In all scenarios below it doesn't match the marketing claims (yet the drive is still at, or close to 100% busy, all the time, for all the tests). The results are still fantastically fast and it is a great SSD, but they don't match the marketing.

          Click image for larger version  Name:	Drive-Performace-Benchmark.png Views:	0 Size:	44.0 KB ID:	50589


          • #6
            Next we can have a look at if we can devise some unlikely scenario that will get us numbers that are close to the WD's marketing numbers. For this we can use the "Advanced Disk Test" benchmark under the Advanced Test menu in PerformanceTest.

            Scenario 1:
            Here we pick a massive block size (8MB), relatively small test file (5GB), and non-random data (which is easy to compress). Not so realistic, but whatever, we don't care at this point.
            Result: 5781MB/sec. So we are getting kind of closer.

            Click image for larger version

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            Scenario 2:
            This time we allow caching of the disk activity (into RAM)

            Success!! We have matched and slightly exceeded the marketing claims of WD. 7432MB/sec read performance!!!

            But wait ,what kind of shenanigans is this? Task manager reports 0% busy time and no disk activity. Conclusion: All the data is being read from RAM and not from disk. Clearly this is a bit of a poor disk test if the disk isn't even being used. What it does illustrate nicely however is the limit on the RAM cache speed. That even fast DDR4 RAM in dual channel can only just hit 7000MB/sec (with the I/O overhead of Disk API calls).

            Click image for larger version

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            Scenario 3:
            This time we use a massive block size (8MB), Asynchronous access (64 queue length), No caching and non-random data & 3 simultaneous threads. No developer would write code to access the disk this way, but whatever.... real life performance this isn't our goal for today.

            This time we have done it for real. 100% load on the disk and 7GB/sec performance!!

            Note that there is still some measurement differences depending on if you think a MB has 1,000,000 Bytes or the traditional 1,048,576 Bytes.
            WD claim a MB has 1,000,000 Bytes, as then their numbers look better that way.
            Also, task manager reports on all disk activity not just from the benchmark. So background activity is also counted.

            Click image for larger version

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