Thursday, August 08, 2019

vSAN Capacity planning - Understanding vSAN memory consumption in ESXi

It is very clear that VMware vSAN (VMware's software-defined storage) has the momentum in the field, as almost all my customers are planning and designing vSAN in their environments. Capacity planning is an important part of any logical design, so we have to do the same for vSAN. Capacity planning is nothing else than simple math, however, we need to know how the designed system works and what overheads we have to include in our capacity planning exercise. Over the years, a lot of VMware vSphere technical designers did get knowledge and practice how to do capacity planning for core vSphere because server virtualization is here for ages (13+ years). But we all are just starting (3+ years) with vSAN designs, therefore it will take some time to gain the practice and know-how what is important to calculate in terms of VMware hyper-converged infrastructure (VMware HCI = vSphere + vSAN).

One of the many important factors for HCI capacity planning is vSAN memory consumption from the ESXi host memory. There is very good VMware KB 2113954 explaining the math calculation and formulas behind the scene. However, we are tech geeks, so we do not want to do the math on the paper so here is the link to Google Sheets calculator I have prepared for vSAN (All-Flash) memory overhead calculation.

Here is the calculator embedded into this blog post, however, it is just in read-only mode. If you want to change parameters (yellow cells) you have to open Google Sheet available on this link.



Note: I did not finish the calculator for Hybrid configuration because I personally believe that 99% of vSAN deployments should be All-Flash. The reason for this assumption is the fact, that Flash capacity is only 2x or 3x more expensive than magnetic disks and the lower price of magnetic disks is not worth to low speed you can achieve by magnetic disks. In terms of capacity, advanced technics like erasure coding (RAID-5, RAID-6) and deduplication + compression can give you back capacity in All-Flash vSAN as these technics are available or make sense only on All-Flash vSAN. If you would like the same calculator for Hybrid vSAN, leave the comment below this blog post and I will try to find some spare time to prepare another sheet for Hybrid vSAN.

Let's document here some design scenarios with vSAN memory consumptions.

Scenario 1
ESXi host system memory: 192 GB
Number of disk groups: 1
Cache disk size in each disk group: 400 GB
Number of capacity disks in each disk group: 4
vSAN memory consumption per ESXi host is 17.78 GB.

Scenario 2
ESXi host system memory: 192 GB
Number of disk groups: 2
Cache disk size in each disk group: 400 GB
Number of capacity disks in each disk group: 2
vSAN memory consumption per ESXi host is 28 GB.

Scenario 3
ESXi host system memory: 256 GB
Number of disk groups: 2
Cache disk size in each disk group: 400 GB
Number of capacity disks in each disk group: 2
vSAN memory consumption per ESXi host is 28.64 GB.

Hope this is informative and it helps broader VMware community. 

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