stuck on low scrub performance

[Tobias Oberstein]

Hi,

I've got a 146TB RAID-Z2 magnetic platter pool which I am scrubbing, but 
performance is very poor:

   pool: tank
  state: ONLINE
   scan: scrub in progress since Tue Jan 17 11:13:29 2017
         5.02T scanned out of 89.4T at 32.2M/s, (scan is slow, no 
estimated time)
0 repaired, 5.61% done

I've already tried modifying various knobs, with no success (I changed 
oids while the scrub is running, without restarting - does that take 
effect?).

I've tried to collect as much info as possible using this script

https://gist.github.com/oberstet/c66baf777ea77ae843b64271df7420a3

with this output

https://gist.github.com/oberstet/a8e12227486ebeb1e85f6f56b880ce7d

---

My expectation from the general performance of this array would be 
scrubbing with 500MB/s (10x more) - this is what we regularily see on 
this array when it goes to the magnetic platters on sequential read loads.

Any helps would be greatly appreciated, as at that speed, the scrubbing 
takes weeks =(

Cheers,
/Tobias