Check with your Application / DBA teams to get memory usage metrics directly from the application / database monitoring tool. A load test might be useful to estimate the right amount of memory that is required for your application. Avoid using OS- or vSphere-based memory metrics for the memory rightsizing task for enterprise application workload.

VMware ESXi host allocates memory upon the start of a virtual machine and has no visibility into the real usage of memory assigned to the Guest OS (due to a self-explanatory security restriction, it’s not allowed to read memory blocks used by the guest OS from the hypervisor layer). The counter Memory consumed will show all touched memory blocks and for modern operating systems we’re expecting that the memory consumed will be close to memory allocated.

Memory active counter estimates the statistical sampling of memory blocks being actively touched through the sampling period (based on soft memory faults) and * should NOT be used* as a performance and/or capacity indicator. It measures the rate of which memory is read or writen. Memory is a form of storage. Using a disk analogy, Memory consumed measures the space used, while active measures the throughput. They measure 2 different dimensions and are not comparable. This counter was not designed for the purpose of rightsizig. For better understanding of various memory counters I would recommend to review the white paper “Understanding Memory Management in VMware vSphere”.

After reading all above it should be clear: application own performance metrics are *the only reliable source of the information* for the “real” memory consumption of the applicatioin.

As a rule of thumb, do not downsize the amount of memory assigned to a VM while migrating to VMware Cloud on AWS. Work with your application / DBA teams to check if adding more memory can be beneficial.

NOTE: VMware vSAN, providing storage resources in VMware Cloud on AWS, is also requiring resources to operate. The table below reflects memory usage by vSAN services for different host types. Ensure that while sizing a VM population on a host, the vSAN overhead is taken into account.

Instance

Memory Overhead (MiB)

Memory Overhead (GiB)

i3.metal

40023

39

i3en.metal 80749 79

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