The vSAN Memory Consumption Calculator has been updated on 19-01-2021 to reflect vSAN 7.x after the KB has been updated and I got some messages that the calculator was not up-to-date anymore.
Disclaimer !! This calculator is officially not supported by VMware, please use this calculator for indications only. Please use the official vSAN sizer at https://vsansizer.vmware.com/ for vSAN sizing.
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:
vSANFootprint = HOST_FOOTPRINT + NumDiskGroups * DiskGroupFootprint
DiskGroupFootprint = DISKGROUP_FIXED_FOOTPRINT + DISKGROUP_SCALABLE_FOOTPRINT + CacheSize * CACHE_DISK_FOOTPRINT + NumCapacityDisks * CAPACITY_DISK_FOOTPRINT
Easy right? Well let me make it even easier for you:
All Flash vSAN Memory Consumption Calculator
Hybrid vSAN Memory Consumption Calculator
* vSAN scales back on its memory usage when hosts have less than 32GB of memory. Source: Link
Hey buddy, I think you’re calculator is wrong. The SSD size is firstly capped at 600GB in terms of the calculation and also your calculator isn’t allowing me to enter more than 7 capacity disks, the formula is total number of capacity drives per node across all diskgroups, not 1 diskgroup.
Doing the calculation myself, in my scenario, all flash, 2x disk groups, 7 capacity disks per disk group, vSAN will consume 33514MB per node.
Hi James,
The vSAN Memory Calculator has been updated. Thanks for the heads up.
Marco
Hi there, your calculation is wrong as it prevents adding more than 7 capacity disks, calculation needs the total number of capacity disks which could be higher than 7 (when 2 or more disk groups are setup)
J
I was kind of floored when I found this out. I understand why, but I have an extremely small lab that I wanted to test VSAN on and pretty much fell out of my chair when I figured out I could only fit 1vm per host with VSAN running.
I’m thinking of moving some drives around to account for this. Using small Msata and SSD drives for Boot and flash and limiting each host to 1 capacity disk each.
if you have less than 32GB of RAM per node, VSAN adapts itself: “vSAN scales back on its memory usage when hosts have less than 32GB of memory”
https://kb.vmware.com/s/article/2113954
Hi Leonardo,
Good catch, I’ve added a note and limited the RAM selection to 32GB.
Marco