In a theoretical system with 16 disks, would it be best to place all 16 on the same 16-port controller, or split them into groups (of 8, or even 4) and run across multiple controllers?  From a performance standpoint.

It really depends on the controllers, board, devices, etc.

ok.  Combo of 5400 and 7200rpm disks, LSI 92xx series, modern gen9 i5-based, and I'm looking for "in general" best practices for maximizing performance.

 

Using rsync to copy from one disk to another, I get 60-80mbs avg, and if I queue up a second copy at the same time, both transfers drop to 9mbs.

 

Assuming no PCIe bottlenecks a 9201-16i is good for around 165MB/s with 16 devices, so it will limit speeds with modern disks, usually capable of around 200MB/s to 250MB/s in the outer tracks, using two 92xx-8i controllers would have no such limit, again assuming no PCIe bottlenecks.

Thank you. 

 

Is it still considered best practice to place parity disks on a separate (in my case onboard) controller?

 

I'm looking at reconfiguring to have two m.2 SSDs for docker and cache, two 7200rpm parity disks on the MB SATA, and 14 disks spread evenly across two LSI HBAs in  x8 slots.

 

Theoretically.

 

Parity can be on the same controller, also don't forget that Unraid array performance can only be as fast as the slowest disk at any point, so if for example you have one slow disk with a max speed of 140/150MB/s the array cannot be faster than that, and a single -16i controller would perform the same as two -8i.