I ran into a strange issue today, a host was consuming way to much of RAM while running a single VM running on it. And that single VM was only configured with half of the RAM of the host?!? After some basic troubleshooting a colleague (thanks Satish) pointed me to the KB article KB2113954 which explained the issue.
The host in question was configured with 128GB of RAM and had 4 disk groups with 7 large capacity disks. If you do the math it required 48GB of RAM to run this vSAN configuration. In other words, it explained what we saw! vSAN was gobbling up more RAM than was anticipated!
To calculate vSAN memory consumption you use this equation:
BaseConsumption + (NumDiskGroups * (DiskGroupBaseConsumption + (SSDMemOverheadPerGB * SSDSize))) + (NumCapacityDisks * CapacityDiskBaseConsumption)
Easy right? Well let me make it even easier for you:
vSAN Memory Consumption Calculator
Note: In the vSAN 6.2 and 6.6 releases, encryption and deduplication features have no impact on memory consumption.