Hi there!
So the server is: HP DL380 G9 Dual Socket Octo Core Intel Xeon E5-2630v3 2.4GHz 128 GB RAM
It's a production server, so I can't "play" with it.
I'll post here some graphs about the SPIKE I had yesterday (2016-02-18) - Note that the spike isn't so big, but usually they are.
Disk utilization during that period
PostgreSQL buffer cache during that period:
Postgres checkpoints: