Why Computers Always Guess the Wrong Copy Time

In the previous part of this series, I explained why small files take longer to copy than large ones. This can make for unpredictable results when you copy a mixed batch of files. I decided to track the time estimates for such a copy and see how wrong the time estimates really are.

Let’s kick off with another animation. I took a series of screenshots when copying files to another disk. I then tracked the estimated time to complete vs the actual completion time. I have also included the raw numbers at the bottom of the page.

Why Computers Always Guess the Wrong Copy Time
Why Computers Always Guess the Wrong Copy Time

How wrong was the original estimate?

The original time estimate was off by almost two hours! 1:59 to be precise. After 9 minutes, the estimate was off by an hour. After around 50 minutes copying, the estimate was only off by 1 minute.

We can’t really use this data to extrapolate much, as it is specific to the data being copied. You’ll notice that the data toward the end of the copy was mostly media files. What the data does show quite clearly is that you cannot trust the time estimates.

General Guide

  • Large media files – Fast copy, Accurate time estimate
  • Small office / text files – Slow copy, Accurate time estimate
  • Mixed data – Mixed copy time, Inaccurate time estimate
Remaining
Estimated Finish
Actual Finish
Wrongness
03:45:00 17:16:00 15:17:00 01:59:00
02:35:00 16:15:00 00:58:00
01:00:00 15:18:00 00:01:00
00:40:00 15:17:00 00:00:00
00:04:00 15:17:00 00:00:00

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