Virtual machines are the one NAS workload where the memory you assign is taken off the top. A VM with 8GB allocated holds 8GB of host memory for as long as it runs, whether the guest is busy or idle, and the NAS has to run its operating system, its file cache and every other app in what remains. This guide gives per-VM figures, a formula, and worked examples for one, two and four VMs.
The short answer
| Setup | NAS memory |
|---|---|
| One Linux server VM (2GB) plus file service | 8GB works, 16GB comfortable |
| One Windows VM (4GB to 8GB) plus file service | 16GB |
| Two VMs, one Windows, one Linux | 32GB |
| Four small VMs or two Windows VMs plus containers | 32GB to 64GB |
| VMs on a ZFS NAS (TrueNAS, QuTS hero) | Add 8GB to 16GB to each figure for the ARC |
The formula
NAS memory = host reserve + sum of VM allocations + headroom.
- Host reserve: 4GB for the operating system, the hypervisor and the usual services. 6GB to 8GB if the NAS also runs Plex, a photo library or several containers.
- VM allocations: what you give each guest. See the table below.
- Headroom: 2GB to 4GB so the file cache survives. On ZFS, 8GB or more, because the ARC is the file cache.
Per-VM allocations
| Guest | Minimum | Comfortable | Notes |
|---|---|---|---|
| Windows Server (domain controller, print, small app) | 2GB | 4GB | Core installs run on less; the desktop experience wants 4GB |
| Windows 10 or 11 desktop | 4GB | 8GB | Browsers and Office push a 4GB VM into swapping |
| Linux server (Debian, Ubuntu Server, Rocky) | 1GB | 2GB | Add memory for the service it runs: a database wants 4GB or more |
| Home Assistant OS | 2GB | 4GB | Add-ons such as cameras or voice raise it |
| pfSense or OPNsense firewall | 1GB | 2GB | More with intrusion detection enabled |
| Database server (PostgreSQL, MySQL) | 2GB | 4GB to 8GB | Performance scales with cache memory |
| Small Kubernetes or Docker host VM | 4GB | 8GB | Depends entirely on what runs inside |
Worked examples
Small office: one Windows domain controller
Host reserve 4GB, Windows Server VM 4GB, headroom 2GB: 10GB. A 16GB NAS runs it with room for the file cache. On a 8GB NAS the file cache shrinks to almost nothing and backups slow down.
Home lab: Windows desktop plus Home Assistant
Host reserve 4GB, Windows 11 VM 8GB, Home Assistant 4GB, headroom 4GB: 20GB. Install 32GB; the next size down, 16GB, leaves no room for containers.
Integrator: two Windows Servers and a database VM
Host reserve 6GB, two Windows Server VMs at 4GB each, PostgreSQL VM 8GB, headroom 4GB: 26GB. Install 32GB, or 64GB if the NAS runs QuTS hero or TrueNAS, because ZFS will use the rest.
What the NAS can take
These figures only work on models with the slots and the maximum to hold them. Two-slot models with 32GB or 64GB maximums are the ones to look at: Synology's Ryzen-based plus and xs lines (DDR4 ECC SODIMM), QNAP's TS-x64, TS-h and TVS-h lines (DDR4 or DDR5 SODIMM and UDIMM), UGREEN's larger DXP units (DDR5 SODIMM), and any TrueNAS board with four DIMM slots. The QNAP, Synology and UGREEN guides list each model's maximum. A NAS with an 8GB ceiling is not a VM host, however good the processor.
Related guides
Can you run virtual machines on a NAS? covers which platforms have a hypervisor. How much RAM for containers covers the lighter path. How much RAM does a NAS need covers every other workload. For server-class VM hosts, the memory types guide and the DDR4 RDIMM collection cover the modules.
Frequently asked questions
Can I give a VM more memory than the NAS has free?
No. The hypervisor refuses to start a VM whose allocation exceeds free memory, or starts it and starves the NAS. Size the allocations to fit with headroom.
Does the VM use all the memory I assign it?
The allocation is reserved from the host whether or not the guest uses it. Some hypervisors can reclaim idle guest memory, but plan as if the full allocation is gone.
How much RAM for a Windows 11 VM on a NAS?
4GB is the minimum Microsoft lists and it runs poorly there. 8GB is the practical allocation. Add 4GB for the NAS and headroom, so a NAS with 16GB runs one Windows 11 VM comfortably.
Do I need ECC for VMs on a NAS?
ECC is about data integrity, not VMs specifically. If the NAS supports ECC, use it; a VM running a database benefits as much as the file service does.
Buying NAS memory for one unit or a fleet
Virtual machines are the one NAS workload where the memory upgrade is not optional. Request a quote with your models and quantities, or contact our sales team. Related: NAS memory hub, business fleet memory and education and government.