Hi there CDV33,
The 5Gb you see in the vSphere client is likely the “active” memory, which is pages on the ESXi host which have recently changed. Now please note this is not unique blocks so cannot be used accurately for sizing memory as it could be very wrong.
To to use your example, it could be that when you look back over the last hour the VM only has 5Gb of active memory during that whole time. This might lead you to think the VM is only using 5Gb however inside the guest it might be changing/modifying/using a different 5Gb of memory every sampling period.
The true measure is to look inside the VM and use those counters.
For Windows guests the ones I like to use from Task Manager are:
The Committed is made up of physical memory and paging file so can be a good indicator of exactly what the guest is using. Add to this check the Free memory as these are pages which are free and immediately available for use. If there is constantly free memory available, then you could likely reduce the memory allocated to the VM as this a high Free memory counter would be indicating that the guest has more memory than it needs.
Does this make sense? If further help required then please post a screenshot of your Task Manager.
