Why is copy speed periodic? (or seems to be)

When copying ~80Gb data from one USB 3.2 stick to another, i noticed that the GUI of the transfer speed had clear periodicity. Out of curiosity, what could be the reason for this? No other active programs were running that, to my awareness, should put a periodic load on the system.

Here's a possibility: Every transfer copies one megabyte and takes 0.4 seconds. The display is updated every second. The counter for "data transferred" is updated when a transfer is complete.

So 50% of the time the display is updated after 2 transfers with 2.0 megabyte completed, and 50% of the time it is updated after 3 transfers with 3.0 megabyte completed. That explains the "periodic" behaviour quite nicely.

In your case, your USB stick might be quite slow, so my numbers are actually realistic. In case of a hard drive: Hard drives have caches. So your computer can write say 100MB per second to the hard drive, but actually only 80MB get written and 20MB fill up the cache. Until the cache is full, and then your hard drive doesn't accept new data until say 100MB of the cache are written to the actual hard drive. That will look to your computer as if write speed wasn't constant.

But since you posted on CS, it's now your duty to explain exactly what happened :-) For example, you could write a program that logs reading and writing with the exact times when they happen.

StackExchange Computer Science Q#145647, answer score: 11