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Disk Linear Write Average Speed - apparent inconsistency


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I'm trying to use the "Linear Write" Disk Benchmark to quantify and visualize the write speed on several different SSDs I'm considering for use for backing up large files -- so linear write performance is exactly what I'm looking for.  I really like the graphs the tool generates because it helps visualize how the drive performs as it fills.  

For the drive in my screenshot below, the average is shown as 1598.3MB/s -- which seems right considering the shape of the graph and my experience with the drive.  However, I was trying to validate the Average speed reported by the tool and came up with some inconsistencies. 

  2042043374_XG6Write.png.4c8fdfb0d36cb99f459f57dae018fa93.png

I'm operating under three main assumptions:

  1. The benchmark writes the full capacity of the drive, no more, no less.
  2. There isn't any idle time between block writes or elsewhere in the test.
  3. The timer is accurately reporting the length of the benchmark

If you take the number of MB written -- the size of the drive -- in this case 976,793.6MB (953.9GB * 1024) and divide that by the time elapsed 1031sec ((17min * 60) + 11) you get 947MB/s.

I performed another iteration of this test and used a camera to record the whole benchmark so I could validate how long it took (I didn't trust myself with a stopwatch), and the time reported by the benchmark ~17min was pretty close to the amount of time measured in my recording... so the issue isn't related to the timer in the benchmark reporting the wrong time.

In Windows I've seen this drive definitely be able to sustain writes over 1.3GB/s (anecdotally), so my calculated 947MB/s definitely doesn't seem right, but then that means that one of my first two assumptions is wrong -- either the timer in the benchmark is including idle time, extending how long it takes to perform the disk write, or the test is performing more writes than the full capacity of the drive.  The latter of these two seems unlikely, but that's just a gut feeling.

--

Can you provide any insight into how the tool calculates the average speed or any thoughts as to why this discrepancy exists?  Is there a way to export the datapoints used for the calculations?

 

Thanks!!

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On 12/31/2020 at 5:00 AM, MrCommunistGen said:

I'm trying to use the "Linear Write" Disk Benchmark to quantify and visualize the write speed on several different SSDs I'm considering for use for backing up large files -- so linear write performance is exactly what I'm looking for.  I really like the graphs the tool generates because it helps visualize how the drive performs as it fills.  

For the drive in my screenshot below, the average is shown as 1598.3MB/s -- which seems right considering the shape of the graph and my experience with the drive.  However, I was trying to validate the Average speed reported by the tool and came up with some inconsistencies. 

 I'm operating under three main assumptions:

  1. The benchmark writes the full capacity of the drive, no more, no less.
  2. There isn't any idle time between block writes or elsewhere in the test.
  3. The timer is accurately reporting the length of the benchmark

If you take the number of MB written -- the size of the drive -- in this case 976,793.6MB (953.9GB * 1024) and divide that by the time elapsed 1031sec ((17min * 60) + 11) you get 947MB/s.

I performed another iteration of this test and used a camera to record the whole benchmark so I could validate how long it took (I didn't trust myself with a stopwatch), and the time reported by the benchmark ~17min was pretty close to the amount of time measured in my recording... so the issue isn't related to the timer in the benchmark reporting the wrong time.

In Windows I've seen this drive definitely be able to sustain writes over 1.3GB/s (anecdotally), so my calculated 947MB/s definitely doesn't seem right, but then that means that one of my first two assumptions is wrong -- either the timer in the benchmark is including idle time, extending how long it takes to perform the disk write, or the test is performing more writes than the full capacity of the drive.  The latter of these two seems unlikely, but that's just a gut feeling.

--

Can you provide any insight into how the tool calculates the average speed or any thoughts as to why this discrepancy exists?  Is there a way to export the datapoints used for the calculations?

Thanks!!

That's a very interesting and peculiar anomaly. My best guess is that the whole process takes more time to complete than what's expected from the average throughput because of the overhead of the graph drawing, time measurement, etc.

If you push a right mouse click on the Save button, you can save all datapoints to a CSV or TSV file ;)

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