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Why do parity check and preclear slow down as they progress?

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I tried to find an answer to this and it seems to be general knowledge that it happens...but why does the read/write speed decrease as a preclear or parity check progresses through the disk. I knew this would happen ahead of time because I have rad other peoples experiences, but I'm just wondering why.


When doing my preclear on 2x 16TB Ironwolf Pro both the zeroing and post read took about 21 hours each. They both started with write/read speeds of about 270MB/s and by the end were going at ~180MB/s for an average of approximately 205MB/s for each stage. Unfortunately when I added them to the new array configuration and started it, I forgot to tick the "parity already valid checkbox" and it automatically started a sync. After that, I decided to make the config again and check the box. But because I could no longer verify the signature on the precleared drives I was paranoid that parity may not be correct so I started a check anyway. Again, the read speeds started at about 260MB/s and now at 75% complete, it is going at 185MB/s. I expect it will creep down further over the next 25%.


Why is this consistently the case?

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