XenServer 6.5 SP1 - 128 GB RAM on Host
Several 32-Bit Linux VM's are deployed, most of them are provisioned with 4 GB or 8 GB of RAM. Once the summarized RAM usage of those 32-Bit VM's reaches 64 GB, no further 32-Bit VM can be started and/or no existing 32-Bit VM can have theyr memory extended. Provisioning an additional 64-Bit VM with whatever RAM value still works fine and results in no problem whatsoever.
It seems that XenServer 6.5 has some sort of Limit like: - If you are going to deploy 32-Bit VM's, you may not exceed 64GB of RAM in total among those VM's.
This is a theory and i am looking for someone with insights to confirm this with reason and/or links to documentations. I have not been able to find the suggested limit in the 6.5 configuration limit pdf.