vSAN Memory Consumption Calculator

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.

vSAN Memory

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.

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Configure vSAN with iSCSI based disks

Every time I needed to test something on vSAN in my vLab, I had to spin up some dusty nested ESXi nodes because I don’t have the proper disks in my test NUCs. But now thanks to William Lam I found a way to allow vSAN to claim iSCSI disks and let them contribute to the vSAN Disk Group! This way I can leave the nested ESXi nodes powered off and very easily test disk and host failures and play around with the Ruby vSphere Console (RVC).

Below you will see a 3 node NUC cluster without any local storage running vSAN! Pretty cool stuff!

Configure vSAN

I really hope that I don’t have to explain this, but then again better safe than sorry. 🙂

Disclaimer !! This is officially not supported by VMware, please do not use this for production or evaluation.

Now that’s out of the way let’s get started.

Configure vSAN with iSCSI disks

Before you continue present the iSCSI LUNs to your ESXi hosts. But be aware that you don’t share the LUNs across the ESXi hosts, present a dedicated set of LUNs for vSAN per ESXi host.

Open the vSphere Web Client and mark one of the presented iSCSI LUNs as a Flash device.
Click on Hosts and Clusters -> Select a Host -> Configure -> Storage Devices -> Select the iSCSI LUN to be marked as SSD -> All Actions -> Mark as Flash Disk.

And voilà! The disk device is now marked as a Flash Disk instead of a Hard Disk.

Repeat the last step for all your ESXi hosts contributing storage to the vSAN Disk Group!

The next step is to SSH to your ESXi host and run the following command to allow iSCSI disks to be claimed by vSAN.


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