No huge RAM cache with HCI
HCI systems most definitely don't have huge RAM caches for two reasons:
1 - RAM is expensive in HCI. The Nutanix storage VM runs in 16-32GB of RAM. If they demanded 128GB for a big cache it would mean they could run fewer VMs on each host. Most VMware environments run out of RAM before CPU. Since VMware and Windows DataCenter Edition (to license Windows guests) cost $15K per host or so MSRP fewer VMs per host makes the whole solution much more expensive.
2 - Power outages. If an HCI system had even 4GB of write cache per node when power failed to the system, and yes I've seen several data center wide power failures generators don't always start like they're supposed to, then all that data would be lost making all the remaining data corrupt. After all if you've written 100GB of data but haven't updated the file system metadata when the system comes back the 100GB of data will be inaccessible.
Storage appliances address this by using NVRAM that detects the power loss and flushes the data to flash using a little battery or capacitor power embedded in the system. If VMware had a UPS monitoring service an external UPS could tell the HCI system that power was failing and have it flush the write cache like an XtremIO does. Unfortunately, unlike Windows or Linux, vSphere has no way to do this.
Biting the hand that feeds IT
