CVM Memory size decrease

Dont do it. CVM size is set during foundation. If you really need to reduce it your clusters is not correctly sized… Add additional memory. But dont reduce the cvm memory. This will create performance problems. 

but it take  64 GB memory  and to reduce it to 32 GB 

Is it an all nvme cluster? 

Be careful with downsizing your memory.. But if you really want to do so… Destroy the cluster and refoundation the cluster with the correct memnory size,

But if you didn’t change the memory siuze during Foundation, it is probably needed…

https://portal.nutanix.com/page/documents/details?targetId=Advanced-Admin-AOS:app-nutanix-cloud-infra-cvm-field-specifications-c.html

as it is suggested, as long as it is production (and you have active support) open a case with support and provide your concern, they will tell you if it is possible (to avoid any performance issue) and they will do it for you.
the calculation for RAM depends on many things one of them is underlying storage type (which ​@JeroenTielen  mentioned it).
so I would say open a case and find out if it is a good call to reduce the CVM memory or not!  :) 

Hello ​@alysho 

In a production Nutanix cluster, I generally wouldn't recommend decreasing the CVM memory size unless it's specifically advised by Nutanix Support. The CVM runs the distributed storage services, and its memory allocation is sized based on factors such as cluster size, storage capacity, enabled features (compression, deduplication, replication, snapshots, etc.), and workload characteristics.

If the goal is to reclaim memory, I'd first verify whether the current allocation exceeds the platform recommendation rather than reducing it proactively. Also check the CVM's actual memory utilization and whether there are any NCC or Prism alerts indicating memory pressure.

If this is Nutanix Community Edition (CE) or a lab environment, the guidance is different, but for supported production deployments it's best to follow the recommended sizing for your AOS version and hardware. If you're unsure why the CVMs were sized as they are, opening a Support case is the safest approach before making changes.