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docs(create-windows-vm): disk bus guidance (virtio-blk default) and Hyper-V enlightenments #1076

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docs(create-windows-vm): recommend virtio-scsi bus and Hyper-V enligh…
abonillabeeche Jul 10, 2026
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Update docs/vm/create-windows-vm.md
abonillabeeche Jul 13, 2026
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abonillabeeche Jul 13, 2026
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65 changes: 62 additions & 3 deletions docs/vm/create-windows-vm.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -68,7 +68,7 @@ The `bootOrder` values need to be set with the installation image first. If you
2. `Type`: Select `disk`.
3. `StorageClass`: You can use the default StorageClass `harvester-longhorn` or specify a custom one.
4. `Size`: The value `32` is set by default. See the disk space requirements for [Windows Server](https://docs.microsoft.com/en-us/windows-server/get-started/hardware-requirements#storage-controller-and-disk-space-requirements) and [Windows 11](https://docs.microsoft.com/en-us/windows/whats-new/windows-11-requirements#hardware-requirements) before changing this value.
5. `Bus`: The value `VirtIO` is set by default. You can keep it or change it to the other available options, `SATA` or `SCSI`.
5. `Bus`: The default value is `VirtIO` (`virtio-blk`), and it is recommended for most Windows workloads. In benchmarking on LVM CSI (Windows Server 2022, with Hyper-V enlightenments enabled), `virtio-blk` matched `virtio-scsi` on sequential and random-read throughput and delivered roughly twice the IOPS at about half the latency on random-write and mixed read/write workloads (databases, write-ahead logs, and backup targets) — in part because only `virtio-blk` can use a dedicated I/O thread. Choose `SCSI` (`virtio-scsi`) when you need many disks behind a single controller or SCSI-specific features such as `UNMAP`/`DISCARD` passthrough and persistent reservations. `SATA` is a fallback for scenarios where paravirtualized drivers cannot be loaded at boot time.
3. The **third volume** is a `Container` with the following values:
1. `Name`: The value `virtio-container-disk` is set by default. You can keep it or change it.
2. `Type`: Select `cd-rom`.
Expand All @@ -82,7 +82,7 @@ The `bootOrder` values need to be set with the installation image first. If you

1. The **Management Network** is added by default with the following values:
1. `Name`: The value `default` is set by default. You can keep it or change it.
2. `Model`: The value `e1000` is set by default. You can keep it or change it to the other available options from the dropdown.
2. `Model`: The default value is `e1000`. This option ensures the guest operating system can obtain network connectivity before paravirtualized drivers are installed. Once VMDP (or VirtIO drivers such as `virtio-win`) is loaded inside the guest, switching to `virtio` provides higher throughput and lower CPU overhead for sustained network transfers.
3. `Network`: The value `management Network` is set by default. You can't change this option if no other network has been created. See [Harvester Network](../networking/harvester-network.md) for the full description on how to create new networks.
4. `Type`: The value `masquerade` is set by default. You can keep it or change it to the other available option, `bridge`.
2. You can add additional networks by clicking `Add Network`.
Expand All @@ -106,7 +106,7 @@ Changing the `Node Scheduling` settings can impact Harvester features, such as d
1. `OS Type`: The value `Windows` is set by default. It's recommended you don't change it.
2. `Machine Type`: The value `None` is set by default. It's recommended you don't change it. See the [KubeVirt Machine Type](https://kubevirt.io/user-guide/virtual_machines/virtual_hardware/#machine-type) documentation before you change this value.
3. (Optional) `Hostname`: Set the VM hostname.
4. (Optional) `Cloud Config`: Both `User Data` and `Network Data` values are set with default values. Currently, these configurations are not applied to Windows-based VMs.
4. `Cloud Config` (Optional): The values for both `User Data` and `Network Data` are set by default. If the selected Windows image includes [Cloudbase-Init](https://cloudbase.it/cloudbase-init/) (a common addition to automated Windows cloud images), the guest processes the `User Data` during its initial boot phase. This allows injection of custom configurations (such as SSH keys, local administrator accounts, driver installation commands, custom hostnames, and first-boot PowerShell scripts) for each virtual machine. If the image does not include Cloudbase-Init, the system still mounts the `User Data` on the configuration drive (`NoCloud` ISO format), but the guest operating system ignores it.
5. (Optional) `Enable TPM`, `Booting in EFI mode`, `Secure Boot`: Both the TPM 2.0 device and UEFI firmware with Secure Boot are hard requirements for Windows 11.

:::note
Expand Down Expand Up @@ -173,6 +173,65 @@ For full instructions on how to install the VMDP guest driver and tools see the

:::

## Recommended Tuning

### Enable Hyper-V Enlightenments

Windows guests benefit significantly from KubeVirt's Hyper-V Top-Level Functional Specification (TLFS) enlightenment features. These include paravirtualized interrupt delivery (SynIC), the Hyper-V synthetic timer with direct interrupts, TLB flush hypercalls, VAPIC, and the Hyper-V clock timer. Enabling these features typically results in the following:

- Reduced initial write allocation time on thin-provisioned storage.
- Stable throughput during sustained write operations and minimal periodic stalls during large file copies.
- Lower per-interrupt scheduling overhead within the guest operating system.

The enlightenments are _not_ enabled by default. To enable them, perform the following steps:

1. On the Harvester UI, go to **Virtual Machines**.

1. Locate the target virtual machine, and then select **⋮ > Edit as YAML**.

1. Add the following block to `.spec.template.spec.domain`:

```yaml
features:
acpi: { enabled: true }
apic: { enabled: true }
smm: { enabled: true }
hyperv:
relaxed: { enabled: true }
vapic: { enabled: true }
spinlocks: { enabled: true, spinlocks: 8191 }
vpindex: { enabled: true }
synic: { enabled: true }
synictimer: { enabled: true }
ipi: { enabled: true }
runtime: { enabled: true }
reset: { enabled: true }
clock:
utc: {}
timer:
hpet: { present: false }
hyperv: { present: true }
pit: { tickPolicy: delay }
rtc: { tickPolicy: catchup }
```

1. Restart the virtual machine to apply the changes.

1. Verify that the enlightenments are active from the cluster level.

```
kubectl get vmi <vm-name> -o json | jq '.spec.domain.features.hyperv'
```

The configuration above matches the recommendation for "best balance between performance and stability" in the SUSE Support Knowledge Base article [Lower disk I/O performance in Windows 11 VMs compared to Linux guests on Harvester](https://support.scc.suse.com/s/kb/Lower-disk-I-O-performance-in-Windows-11-VMs-compared-to-Linux-guests-on-Harvester).

You can enable more aggressive enlightenments (such as `tlbflush`, `frequencies`, `reenlightenment`, or `synictimer` with `direct: true`) on individual virtual machines to improve their performance. However, these enlightenments have the following disadvantages:

- Require explicit support from the underlying cluster hardware.
- Can affect live-migration compatibility across cluster nodes that run on different CPU generations.

For more information, see [HyperV optimizations](https://kubevirt.io/user-guide/user_workloads/guest_operating_system_information/#hyperv-optimizations) in the KubeVirt documentation and [Tuning Windows VM Performance on SUSE Virtualization](https://www.suse.com/c/tuning-windows-vm-performance-on-suse-virtualization/) in the SUSE blog.

## Known Issues

### Windows ISO unable to boot when using EFI mode
Expand Down
50 changes: 47 additions & 3 deletions versioned_docs/version-v1.8/vm/create-windows-vm.md
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -68,7 +68,7 @@ The `bootOrder` values need to be set with the installation image first. If you
2. `Type`: Select `disk`.
3. `StorageClass`: You can use the default StorageClass `harvester-longhorn` or specify a custom one.
4. `Size`: The value `32` is set by default. See the disk space requirements for [Windows Server](https://docs.microsoft.com/en-us/windows-server/get-started/hardware-requirements#storage-controller-and-disk-space-requirements) and [Windows 11](https://docs.microsoft.com/en-us/windows/whats-new/windows-11-requirements#hardware-requirements) before changing this value.
5. `Bus`: The value `VirtIO` is set by default. You can keep it or change it to the other available options, `SATA` or `SCSI`.
5. `Bus`: The value `VirtIO` (`virtio-blk`) is set by default and is recommended for most Windows workloads. In benchmarking on LVM CSI (Windows Server 2022, with Hyper-V enlightenments enabled), `virtio-blk` matched `virtio-scsi` on sequential and random-read throughput and delivered roughly twice the IOPS at about half the latency on random-write and mixed read/write workloads (databases, write-ahead logs, and backup targets) — in part because only `virtio-blk` can use a dedicated I/O thread. Choose `SCSI` (virtio-scsi) when you need many disks behind a single controller or SCSI-specific features such as UNMAP/DISCARD passthrough and persistent reservations. `SATA` is a compatible fallback for scenarios that cannot load paravirtualized drivers at boot time.
3. The **third volume** is a `Container` with the following values:
1. `Name`: The value `virtio-container-disk` is set by default. You can keep it or change it.
2. `Type`: Select `cd-rom`.
Expand All @@ -82,7 +82,7 @@ The `bootOrder` values need to be set with the installation image first. If you

1. The **Management Network** is added by default with the following values:
1. `Name`: The value `default` is set by default. You can keep it or change it.
2. `Model`: The value `e1000` is set by default. You can keep it or change it to the other available options from the dropdown.
2. `Model`: The value `e1000` is set by default so the guest can obtain network connectivity before paravirtualized drivers are installed. Once VMDP (or virtio-win drivers) is loaded inside the guest, switching to `virtio` provides higher throughput and lower CPU overhead for sustained network transfers.
3. `Network`: The value `management Network` is set by default. You can't change this option if no other network has been created. See [Harvester Network](../networking/harvester-network.md) for the full description on how to create new networks.
4. `Type`: The value `masquerade` is set by default. You can keep it or change it to the other available option, `bridge`.
2. You can add additional networks by clicking `Add Network`.
Expand All @@ -106,7 +106,7 @@ Changing the `Node Scheduling` settings can impact Harvester features, such as d
1. `OS Type`: The value `Windows` is set by default. It's recommended you don't change it.
2. `Machine Type`: The value `None` is set by default. It's recommended you don't change it. See the [KubeVirt Machine Type](https://kubevirt.io/user-guide/virtual_machines/virtual_hardware/#machine-type) documentation before you change this value.
3. (Optional) `Hostname`: Set the VM hostname.
4. (Optional) `Cloud Config`: Both `User Data` and `Network Data` values are set with default values. Currently, these configurations are not applied to Windows-based VMs.
4. (Optional) `Cloud Config`: Both `User Data` and `Network Data` values are set with default values. If the selected Windows image ships with [Cloudbase-Init](https://cloudbase.it/cloudbase-init/) (a common addition to modern Windows images), the `User Data` is processed at first boot and can inject per-VM configuration — SSH keys, local administrator accounts, VMDP install commands, hostname, first-boot PowerShell scripts, and so on. If the image does not include Cloudbase-Init, the `User Data` is still stored on the noCloud drive but nothing inside the guest will consume it.
5. (Optional) `Enable TPM`, `Booting in EFI mode`, `Secure Boot`: Both the TPM 2.0 device and UEFI firmware with Secure Boot are hard requirements for Windows 11.

:::note
Expand Down Expand Up @@ -173,6 +173,50 @@ For full instructions on how to install the VMDP guest driver and tools see the

:::

## Recommended Tuning

### Enable Hyper-V Enlightenments

Windows guests benefit substantially from KubeVirt's Hyper-V TLFS (Top-Level Functional Specification) enlightenment set — paravirtualized interrupt delivery (SynIC), the Hyper-V synthetic timer with direct interrupts, TLB flush hypercalls, VAPIC, and the Hyper-V clock timer. Enabling them typically:

- Reduces first-touch write allocation time on thin-provisioned storage.
- Smooths out sustained-write throughput and reduces periodic stalls during large file copies.
- Lowers per-interrupt scheduling overhead inside the guest.

The enlightenments are not enabled by default in the VM creation form. To apply them, click **Edit as YAML** in the VM edit view and add the following to `.spec.template.spec.domain`:

```yaml
features:
acpi: { enabled: true }
apic: { enabled: true }
smm: { enabled: true }
hyperv:
relaxed: { enabled: true }
vapic: { enabled: true }
spinlocks: { enabled: true, spinlocks: 8191 }
vpindex: { enabled: true }
synic: { enabled: true }
synictimer: { enabled: true }
ipi: { enabled: true }
runtime: { enabled: true }
reset: { enabled: true }
clock:
utc: {}
timer:
hpet: { present: false }
hyperv: { present: true }
pit: { tickPolicy: delay }
rtc: { tickPolicy: catchup }
```

Restart the VM for the changes to take effect. Verify from outside the guest:

```
kubectl get vmi <vm-name> -o json | jq '.spec.domain.features.hyperv'
```

The set above matches the "balanced set for performance and stability" recommended in the SUSE Support KB article [_Lower disk I/O performance in Windows 11 VMs compared to Linux guests on Harvester_](https://support.scc.suse.com/s/kb/Lower-disk-I-O-performance-in-Windows-11-VMs-compared-to-Linux-guests-on-Harvester). More aggressive enlightenments (`tlbflush`, `frequencies`, `reenlightenment`, or `synictimer` with `direct: true`) can be enabled per-VM when the cluster hardware supports them, but they may affect live-migration compatibility across nodes with different CPU generations — see the [KubeVirt Hyper-V enlightenments documentation](https://kubevirt.io/user-guide/compute/hyperv/) and the [SUSE Virtualization blog on tuning Windows VM performance](https://www.suse.com/c/tuning-windows-vm-performance-on-suse-virtualization/) for details.

## Known Issues

### Windows ISO unable to boot when using EFI mode
Expand Down