Question regarding second parity performance


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Today I had to replace the second parity disk in a 21 data disks array on 6.8.1. Both parity disks are 6TB. The largest data disk is 3TB.

 

Something weird is going on during the still running rebuild. As long as data disks were involved performance was over 100MB/s. After 3TB (size of the largest data disk) performance dropped to under 80MB/s immediately. 

 

I mean: Frst parity disk is being read and second parity disk is being written. That's all. At 80MB/s. I don't get that. Reading 21 data disks plus 1 parity disk and writing 1 parity disk in parallel is much faster than reading just one and writing just one? Both parity disks are in perfect shape. 

 

Who can explain that? Never seen that before. Last time I had to do that was with 6.7.* and it was way faster. In fact, when the rebuild crossed the largest data disk parity rebuild got a huge boost with 6.7.*.

 

Many thanks in advance.

 

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