ESXi is on SD card (HP custom image)
VMs, scratch and logfiles - on SSD
Additional virtual storage disks stored on HDDs
As soon as vmdk on HDD assigned to VM and VM started (in BIOS), ESXi every other second use HDD with some small writes.
Esxtop shows zero disk activity from VM, but shows disk activity from ESXi itself.
If VM is shutted down, ESXi stops using disk.
It is quite annoying that otherwise inactive disks rattling all the time.
If this is some monitoring stuff, can it be disabled or changed monitoring interval?
One possible explanation comes to my mind: If your VM does not have 100% vram reserved, ESXi creates .vswp (or .vmem?) file, which is basically mirror of VM-ram (size equal to non-reserved amount). This file is periodically updated, as ram of running VM is being modified continuously.
This if perfectly normal behaviour, but if it really disturbs you, I recommend to start VM with 100% vRAM reservation, with no RAM over-commiting. Then the above mentioned VM-file is not created at all. There are other files too which are being updated when VM is running and could cause disk-writes (log-files, performance-counters, etc) but it should not happen so frequently as updating VM swap-file...
All swaps/cache/logs and VMs themself reside on SSD which doesn't rattle:)
But ESXi rattling HDD even if VM is booted to BIOS. I've tried now to start VM with 100% memory reservation and it doesn't change anything.
I think it is related to some performance monitoring, journaling or storage adapter bus scanning/polling intervals (or glitching HP driver).