Nutanix has a very smart architecture for their platform – standard hardware technologies combined in a very smart way. And the glue between is the Controller VM. This design allows full control of the ressources of the software-defined storage architecture (well Nutanix ECP is much more than a SW defined storage approach). One key ressource is the RAM of the CVM which can be changed as required without the need of any vMotion/Live Migration task.
Recently there was a discussion on the NTC (Nutanix Technology Champion) slack channel regarding CVM memory size when using different types of data reduction techiques. Michael Webster gave us some answers on open questions. The findings are summarized in the list below. I have also added some general guidelines just for completeness:
- CVM RAM sizing is the same for every supported hypervisor of Nutanix
- All CVMs in a cluster must have the same amount of RAM
- Default and minimum is 20GB RAM per CVM
| Platform | Default Memory (GB) |
|---|---|
| Default | 20 |
| Storage Heavy | 28 |
| Storage Only | 28 |
| Large server, high-performance, all-flash | 32 |
- With Inline dedup add 8GB RAM
- Post dedup add 12GB RAM (requires that inline dedup is also activated)
- EC-X (erasure coding) doesn’t require additional RAM
- Compression (inline and postprocess) doesn’t require additional RAM
Additional ressources:
- Rene Van Den Bedem wrote a blog about CVM sizing guidelines http://vcdx133.com/2015/12/18/npx-controller-vm-sizing
pichlbauer 
Always is reccomended at least 20 GB RAM to can provide more space for cache, you can find this information in Nutanix KB
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