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I use Ubuntu 17.04, though this problem was persistent in many previous versions. Long ago (a year or two, I think) I started to spot it.

Immediately (no more than a couple of minutes) after the system start up the system load indicator is starting to report writing to the disk, as it is shown on the next screen shot (sorry for a photo, since I don't know how to make a screenshot with the dropdown window visible)

unknown process is writing to the disk

As you can see, the disk write is being reported as 1.5 MB/s (though it is often 1-2 GB/s) and the iotop program reports no writing at all. Then, after five or ten minutes everything turns back to normal.

I tried to search for this, but advices which were given in this question seem to yield no result in my case, like the advice to use iotop.

Does anyone know what is going on here and should I be worried? Thank you.

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Your system is under heavy load (99.24%) from:

fstrim (8)           - discard unused blocks on a mounted filesystem

which is an obvious source of writes to the disk.

On the other side, the fact that a rate goes very high doesn't mean that there is a lot of writting. If you write 100 bytes in 1 nanosecond, you will get a rate of 100 Giga Bytes per second. But you actually just write 100 bytes. that is not exactly what happens in what is being reported due to averages and other issues, but you should get the idea of why a rate could be very high.

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