Maybe I'm doing something horribly wrong. My setup was:

- 6x4TB HDD

- 2x120GB SSD

- OSD per disk

- k=4,m=2 erasure coded HDD pool

- 1 copy replicated (not) SSD pool

- SSD pool writeback cache for HDD pool

I was consistently seeing the SSD writeback cache fill up then the disks thrashed at 100% IO utilization, limiting incoming writes to ~40MB/s from memory.