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Parity write speed and USB3 questions

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Hi all!


I've just installed the trial on an 8-drive system. The write speed to the array is around 100MB/s for the first 10 seconds of a transfer, then it drops to 45-50-MB/s.



1) I assume this is when the parity overhead kicks in? The parity drive is the same make, model and capacity as the array drives. Do I understand this correctly?

2) Would this improve if I replaced the parity HDD with a same capacity SSD? I do not have enough SATA connectors for a separate cache drive I'm afraid

3) Can a cache drive be connected via USB3 and be fully recognised and utilised by Unraid? I would have to swap the motherboard, hence solution 2) would be easier for me


Thank you everyone kindly in advance for replies and guidance!


Best wishes, 



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4 hours ago, robinkoehler said:

The write speed to the array is around 100MB/s for the first 10 seconds of a transfer, then it drops to 45-50-MB/s.

This is the network speed writing to the RAM I/O buffer initially, then when buffer is full it drops to write speed of the parity array. The somewhat slower write speed is typical due to realtime parity updates. Doubt SSD parity will improve this significantly. See here for more on the two different methods for updating parity:




USB not recommended for array or pool (cache) disks due to unreliable connections.


You can mix disk sizes in the array. Parity must be at least as large as the largest single data disk. Why do you think you can't spare a port for cache? You could just use larger disks to get the same capacity.


There are other benefits to cache besides user share write speed. You can also get better performance for docker/VM by keeping them on SSD cache.

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Hi trurl, 


Thank you for the reply! Ok dang, that's unfortunate, but it makes sense. 


I would like to use all 8 ports for storage, and currently do not have the budget for larger capacity drives. 


You've answered everything I needed to know at this point though, much appreciated!


Best wishes, 



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