Estimated memory requirements for ESXi hosts running with very large memory reservations by single or multiple virtual machines
Article ID: 420935
Updated On:
Products
Issue/Introduction
When deploying virtual machines (VMs) with very large memory reservations (e.g., for in-memory databases, or VMs with full memory reservation required), you may encounter a challenge in accurately estimating the ESXi host's non-VM overhead memory usage. Failing to allocate sufficient memory for the ESXi host's own operations can lead to the last VM started failing to power on due to an inability to allocate the reserved memory, even if the total physical memory appears sufficient.
The ESXi host memory consumption (or overhead) is not static and cannot be determined upfront. It is influenced by the physical server configuration and enabled vSphere features. This article provides general guidelines for estimating a baseline requirement from a base ESXi install, along with estimating when there are additional vSphere features are enabled.
Environment
ESXi 8.x, 9.x
Cause
The ESXi hypervisor, kernel modules, device drivers, and specific vSphere features (like vSAN or NSX) consume a substantial and configuration-dependent amount of physical memory. When VMs with large reservations are powered on, they claim their reserved memory immediately. If the combined total of the reserved VM memory plus the ESXi overhead exceeds the physical memory, the last VM's attempt to secure its reservation will fail.
It may also lead to, not limited to, behaviours such as: Not enough memory for base modules of ESXi causing host to be unmanagable, not enough memory non-base modules preventing them to work correctly, VMs trying to access memory region reserved for other processes/modules/VMs that would cause their OS to crash since they can't access the originally deemed to be available memory.
Resolution
To mitigate the risk of startup failure after an ESXi memory bottleneck and/or VMs getting powered off due to lack of free memory available, reserve an appropriate amount of physical RAM for the ESXi host overhead. The following table provides estimated guidelines based on the number of physical CPU sockets and the total installed memory, using any workloads either as a single VM or multiple VMs totalling towards amount of memory reserved:
| Physical Host CPU Sockets | Estimated ESXi Host Memory Requirement (Guidelines) | Example Memory Requirement for ESXi Host based on Total Installed Memory Reserved by Single or Multiple VMs |
| 2 | 64GB – 256GB (Default: 128GB) | 2TB Installed Memory --> 128GB |
| 3TB Installed Memory --> 192GB | ||
| 4 | 128GB – 384GB (Default: 256GB) | 4TB Installed Memory --> 256GB |
| 6TB Installed Memory --> 384GB | ||
| 8 | 256GB – 768GB (Default: 512GB) | 8TB Installed Memory --> 512GB |
| 12TB Installed Memory --> 768GB |
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