RAID-5 vs RAID-6 rebuild speed

I have been upgrading disks in my various ReadyNAS units.  The 4 bay units are RAID-5 and the 6 bay units are RAID-6. 

I haven't really seen it discussed here, so I thought I would point out that if you're using RAID-6, the rebuild times are going to be a lot longer than if you're used to RAID-5, due to having to generate the additional parity.    

For example, I have my RN204 and one of my RN316's. 

• Both are using 7200 RPM drives. 
• Both are going from 4TB to 10TB drives.
• Both are using FlexRAID.
  
  

The rebuild speed on the RN204 using RAID-5 is about 95MB/s, whereas the RN316's using RAID-6 is about 30MB/s. I was surprised that the RN316 with its 2 core/4 thread Atom CPU was that much slower than the little dual core ARM in the RN204. I figured that it would be more linear. But, If I remember correctly, my RN516 with its 8 core 2.4GHz Xeon and 16GB of memory was also about 30MB/s using the same 7200RPM drives, it's possible that ReadyNAS OS is artificially limiting the rebuild speed (providing my memory is indeed correct).


So nothing earth shattering here, just something I found interesting and thought I'd share.