Migrating a Windows boot disk from an IDE connection to Virtio within a KVM environment is a crucial step for significantly improving virtual machine performance. While installing Windows directly onto a Virtio disk is the ideal scenario, existing VMs often start with the slower IDE interface. The challenge is that Windows requires the appropriate Virtio drivers to be loaded before it can boot from a Virtio disk, and these drivers are not present or active when the system is initially configured for IDE.
The most effective method to achieve this migration involves a clever workaround: introduce the Virtio drivers to the running Windows system while it’s still booting from the IDE-attached disk. This typically requires attaching the Virtio driver ISO image to the VM.
The process generally begins by shutting down the VM. Next, you add the Virtio driver ISO to the VM’s virtual CD-ROM drive. Critically, you then need to configure the Windows boot disk to be presented to the VM using both an IDE interface (for the initial boot) and a Virtio interface simultaneously. This dual attachment allows Windows, once booted from IDE, to detect the same physical or virtual disk being presented via the Virtio interface.
Upon starting the VM and booting into the Windows operating system using the IDE interface, you can then access the attached Virtio driver ISO. From the ISO, you install the necessary Virtio drivers for the storage device (often vioscsi or viodisk). Installing these drivers while Windows is running allows the system to properly recognize and communicate with Virtio devices before attempting to boot from one.
Once the Virtio drivers are successfully installed and Windows recognizes the disk via its new Virtio path, you shut down the VM again. The final step is to reconfigure the VM’s storage settings to present the Windows boot disk only via the Virtio interface, removing the IDE connection for that specific disk.
When the VM is powered back on, Windows will now attempt to boot from the disk attached via the Virtio interface. Because the Virtio drivers were correctly installed in the previous steps while the IDE connection was active, Windows has the necessary components loaded to handle the Virtio disk from the very start of the boot sequence. This results in the VM booting with and utilizing the significantly faster Virtio block device driver, leading to improved disk I/O performance for the operating system and applications.
Source: https://kifarunix.com/change-windows-boot-device-from-ide-to-virtio-on-kvm/
