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What is the explanation of the following phenomenon observed in local file copy speed?

enter image description here

As you can see, it keeps stable at 120 MB/s, but at the beginning, it was nearly double of that.
Is there some kind of "bandwidth-shaping" in place?

In this specific case, the file is copied between folders on the same internal mSATA SSD drive.

I can consistently reproduce this, even with much higher initial speed:

enter image description here


The suggested duplicate answer indicates that initial speed bump could be caused by caching. But this seems unlikely as the sole reason because:

  1. if it was only caching, the troughput decrease would be quite steep (after cached content got exhausted), but we are observing relatively slow and fluent decrease

  2. if it was only caching, it could hardly cover initial 3 GB of transferred data until the speed settles down. 3 GB is too much for caching. Machine RAM is 8 GB, used is 6 GB, extra 1 GB is occupied during the entire copying process.

  3. I can observe the same initial speed bump even with caching disabled:
    enter image description here
    Without caching, the average speed is consistently half of the speed seen with cache. This is the evidence that caching accelerates the entire copying process, so initial high speed must be attributed to something different.

So there must be something else. What it is?

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  • I've been wondering the same thing lately hopefully a good explanation can be found!! Mar 5, 2016 at 10:15
  • Possible duplicate of What do different patterns mean in Windows 8 file copy dialog
    – DavidPostill
    Mar 5, 2016 at 10:20
  • @DavidPostill – I don't think that suggested duplicate answer covers this better than giving (incorrect) guess about caching. There must be something more interesting going on. Please see the updated question.
    – miroxlav
    Mar 5, 2016 at 12:08
  • Most likely it is the initial data burst supported by the sata specification. Advertised sata speeds are burst speeds, not continuous data transfer speeds.
    – Moab
    Mar 5, 2016 at 15:06
  • 2
    Burst speed is from the hard drive on board cache to the platters, it cannot be sustained. All Sata advertised speed specifications are theoretical, not real world....en.wikipedia.org/wiki/Serial_ATA
    – Moab
    Mar 5, 2016 at 18:22

1 Answer 1

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Check out the comment link to see the possible duplicate thread, if you don't believe this was due to caching, you can turn cache off under device manager and test it again, because by default Windows is enabled caching for local fixed disk

enter image description here

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  • Thank you! You helped me to confirm this is NOT caused by caching. I can observe it even with caching OFF. The question is now updated with findings.
    – miroxlav
    Mar 5, 2016 at 12:06

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