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I have started read-scan the disk using:

smartctl -t select,10-11 -t afterselect,on -t pending,45 /dev/sda

It seems, however, it kills the disk performance. I had expected that the read-scan would (just like the tests) be suspended when there was disk activity.

I had also expected the read scan (like the tests) would run without load on the server, but according to iostat -x -d 1 it seems read-test goes through the controller.

Is this normal? Can I suspend the read scan when the disk is active (e.g. only run read scan if idle for 500 ms)?

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As per smartctl man page, you can abort non-captive test with smartctl -X /dev/sda. According to that page, your selective test is non-captive, because you didn't use the -C option.

But note that all tests do harm performance. Although they run only when the disk is idle for some time (I suspect some miliseconds), it still moves the disk heads to another position, so the disk has to seek more for real data activity. On busy server I am able to run only short tests, because the long test would take weeks and the performance would be impacted for the whole time.

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