Latency spike for iSCSI

[datnus]

Hi everyone,
I use FreeNAS 8.3 with SSD as ZIL and 9 HDD raidz2 (1 spare).
The performance is quite ok in term of latency via ESXi.
However, there are latency spikes periodically.
Blue line is for read and red line is for write latency.

How should I prevent those latency spikes?
Thanks so much.

 

[datnus]

I believe this happens sharply when the L2ARC is out of space.
In normal cases, the spikes are only up to 100-200 ms.