DD test speed dips over time during burn-in

I am setting up a new system and wanted to burn-in the HDDs before use. I ran the following command on each drive in parallel.

dd if=/dev/sdx of=/dev/null bs=1M status=progress
This is what it looks like:
https://imgur.com/a/elfWme0

Is it normal for the speed to dip over time? (The dips at the start is me cancelling the test and figuring things out).
It started at 249MiB/s now at 173MiB/s

The drives are: ST12000NT001

This might be the most normal thing but I am not knowledgeable enough to know if it is or not.

Thanks

The drives are CMR, so that is not an issue.
The image, please post the image on these forums. Many people, including myself will not go to another site primarily due to viruses. Also, images on other links tend to go away after time, ours will remain with the forum forever, which really helps when someone read this thread next year for example.

Without knowing more about your system, I can’t tell you why it dipped, but typically it shouldn’t. I suspect you have an overheating issue. But post the image here, let’s take a look. Also post all your hardware specs.

Feel free to read the link below called Joes Rules. It will help you post a good question and provide you the information you may need to provide us.

I tried! I couldn’t post a link and couldn’t upload a picture.

oh no way. Now it worked. It kept saying:
“An error occurred: Sorry, you can’t embed media items in a post.”

New users can’t post images as an anti-spam measure. Sorry about that :stuck_out_tongue:

@kettle yes, this “gradual slope downward” is a very normal behavior for hard drives, it’s a result of the constant angular velocity of the physical platter. Because the areal density is the same, you’re covering more sectors in the same amount of time on the outer edge of the platter.

Physics!

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Pretty much normal. There are more sectors around the disk in the outer cylinders than in the inner cylinders, which means more bytes per second pass under the heads.

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I am nerding out at that explanation. Fantastic!
Thank you all.

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