Hi,
I think we met similar problems although in different environments.
My problem was on a low-end server: HP Proliant ML315, 1xXeon, 16GB RAM, 2x1TB Seagate SATA3, plus a Plextor SSD PCIe for SLOG and L2ARC, embedded SATA RAID card in AHCI mode.
When system IO was just idle, a qm snapshot...