# Proxmox host memory: what Windows guests really cost

> Measured host memory on a 31.2GB Proxmox box running two Windows 11 VMs, why the numbers are larger than the guest allocation, and how to plan headroom.

**Tested on**

- Build: 26300.9457 / 26100.1742
- Edition: Pro (26H2) / LTSC 2024
- Architecture: x64
- Evidence: E2, M-V
- Verified: 2026-10-29

## The short answer

Guest RAM is not host RAM. A "32GB host" running two 8GB Windows VMs does **not** have 16GB free.

In this site's lab — Proxmox VE 9.2.2, 16 cores, 31.2GB RAM, two Windows 11 VMs at 8GB each — the host reported roughly:

| Metric | Observed |
|---|---|
| Used | ~24GB |
| Available | ~6.7GB |
| Swap used | ~4.7GB |

That is the real number to plan with. Two 8GB guests consumed 16GB of guest allocation and about 24GB of host memory.

## Where the extra memory goes

| Consumer | Why |
|---|---|
| **Guest working set** | Windows uses what it is given; 8GB guest often holds 6–7GB |
| **QEMU/KVM overhead** | each VM process, device models, OVMF, vTPM |
| **ZFS ARC** | the filesystem cache will take a large share if left unbounded |
| **Host services** | PVE daemons, container(s), monitoring |
| **Swap** | the safety valve that tells you the host is over-committed |

ZFS ARC is the one people forget. On a ZFS root, ARC grows to fill RAM by default; it is cache, so it shrinks under pressure — but it also means the "free" figure is misleading and swap can be used at times you did not expect.

## Measure it properly

On the PVE host:

```bash
free -h
zfs_arc_summary 2>/dev/null | head -20   # or: cat /proc/spl/kstat/zfs/arcstats | head
qm list                                   # VM inventory
qm config 361 | grep -E 'memory|balloon|cores'
```

Then per-VM, from inside the guest:

```powershell
Get-CimInstance Win32_OperatingSystem |
  Select-Object @{n='TotalGB';e={[math]::Round($_.TotalVisibleMemorySize/1MB,1)}},
                @{n='FreeGB';e={[math]::Round($_.FreePhysicalMemory/1MB,1)}}
Get-Counter '\Memory\Available MBytes' -MaxSamples 3
```

If the host swap is being used while guests report plenty of free memory, the host is over-committed — not the guests.

## Planning headroom

A rule that survives contact with reality:

- reserve **2–4GB** for the host (PVE + ZFS ARC floor + monitoring);
- allocate **guest RAM × 1.2** as the real cost (overhead + working set drift);
- leave **10–15%** unused;
- never plan to the last gigabyte.

For a 32GB host: `32 − 3 = 29GB` usable, so **two 8GB guests plus one 8GB guest is already at the edge**, and a third Windows VM means freeing memory first — exactly the conclusion in this lab's record.

## Tuning that helps (and does not)

| Change | Effect |
|---|---|
| Bound ZFS ARC (`zfs_arc_max`) | frees RAM predictably; slightly less cache |
| Set `balloon` on guests | host can reclaim under pressure; guest may slow |
| Reduce guest RAM to 6GB | works for light workloads, not for Windows Update + browser |
| Disable memory compression in guest | frees nothing useful, usually hurts |
| Add a second host | the only real fix once you are at the limit |

> **Caution: Over-commitment shows up as unexplained slowness**
>
> When the host swaps, every guest pauses during page-in. The symptom inside Windows is "random slowdowns with no CPU load" — easy to misdiagnose as a driver or storage problem. Check host swap first.

## What this observation does and does not prove

- **Proves**: on this host, with these guests, two 8GB Windows VMs cost ~24GB of host memory and pushed the host into swap.
- **Does not prove**: that every 32GB host behaves this way — ZFS settings, guest workloads and PVE version all change the number.

Re-measure your own host; the method above is the point.

## Related

- [Mini PC as a PVE host](https://windows.homes/en/hardware/homelab/mini-pc-host/)
- [RAM sizing](https://windows.homes/en/hardware/ram/)
- [Memory compression and page file](https://windows.homes/en/hardware/ram/compression-pagefile/)

## Lab boundary

E2 (M-V): host figures come from the lab record for this two-VM configuration. Guest-side counters were not sampled simultaneously for this page, so only the host observations are quoted as measured.

## Sources

- [Proxmox VE — Windows 11 guest best practices](https://pve.proxmox.com/wiki/Windows_11_guest_best_practices)
- [Proxmox VE — Storage](https://pve.proxmox.com/wiki/Storage)
- [Microsoft Learn — Windows 11 requirements](https://learn.microsoft.com/en-us/windows/whats-new/windows-11-requirements)

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Source: https://windows.homes/en/hardware/homelab/host-memory/