For an ordinary disk in a modern QEMU virtual machine, use virtio-blk if the guest image has a working VirtIO driver. If it does not, select a controller the guest already supports—QEMU identifies AHCI SATA as a widely supported fallback for modern x86 guests. The choice depends on the device the guest can use and the features you need, not whether the disk image is raw or qcow2.
What “HDA” and “virtio-blk” actually mean
They are not two names for equivalent controllers. In this comparison, “hda” refers to QEMU’s legacy -hda command-line shortcut and the older IDE-style assumptions that often accompany it. QEMU 8.1 describes -hda as shorthand that expands into -drive options, with legacy PC assumptions; it is not recommended for modern configurations. QEMU says explicit -device and -blockdev configuration is the only interface guaranteed to remain stable for management tools and scripting. See the QEMU 8.1 invocation reference.
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virtio-blk is a paravirtualized block-device model: QEMU presents a disk device to the guest, which must have a suitable VirtIO driver to access it. QEMU describes VirtIO devices as designed to be efficient to emulate and virtualize in its current VirtIO documentation, identified as QEMU 11.1.50 at the time of access.
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Choose based on the guest and the job
| Situation | Practical choice | Why |
|---|---|---|
| Ordinary disk; guest has a working VirtIO driver | virtio-blk |
QEMU recommends VirtIO storage and describes virtio-blk as a high-performance disk choice with a thin software stack. |
| Guest cannot currently access a VirtIO boot disk | Use a controller the guest already supports, then plan driver setup before switching | QEMU identifies AHCI SATA as widely supported by modern x86 operating systems when VirtIO drivers are unavailable. |
| Need CD-ROM devices, full SCSI commands or passthrough, or many disks | virtio-scsi |
It provides full SCSI support and supports many LUNs; virtio-blk is for disks and does not support non-disk devices such as CD-ROMs. |
Working from an old example using -hda |
Treat it as legacy syntax; use explicit device and backend configuration in modern tooling | The shortcut carries legacy assumptions and is not QEMU’s recommended modern configuration interface. |
QEMU’s 2021 guidance recommends considering virtio-scsi for more than 28 disks or when full SCSI support is required. The disk-count guidance is framed around PCI-slot constraints on a typical i440fx configuration; 28 is not a universal limit. See QEMU’s virtio-blk and virtio-scsi configuration guide.
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When virtio-blk is the better fit
Choose virtio-blk for straightforward virtual disks when the guest can load its driver and you do not need SCSI-specific functions. QEMU recommends VirtIO storage over emulated storage controllers as generally more performant and fully featured in QEMU, and describes virtio-blk as a high-performance option because it has a thin software stack. That is QEMU’s recommendation, not a measured speedup guaranteed for every workload or VM.
VirtIO drivers are available for Linux, Windows, and other operating systems, but availability in general does not prove that a particular image or kernel has the needed driver enabled. Check the guest image and its OS-specific driver support before assigning virtio-blk as its boot disk. QEMU recommends current drivers for fixes and performance improvements; its VirtIO documentation provides the general device context.
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When virtio-scsi makes more sense
Virtio-scsi is the better choice when the VM needs full SCSI functionality rather than only ordinary disks. It can support SCSI devices such as CD-ROMs and is suited to SCSI passthrough or configurations with many disks. QEMU’s configuration guide recommends it when full SCSI support is required, and advises it above 28 disks in the typical i440fx PCI-slot context described there.
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For a disk-only setup without those requirements, virtio-blk is the simpler fit. The right choice is not determined by a blanket claim that one bus is always faster; use the device features your guest needs.
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- Flawless 4K@60Hz Video with HDMI: Delivers exceptional clarity and smoothness with its 4K@60Hz HDMI port, making it ideal for high-definition presentations and entertainment. (Note: Only the HDMI port supports video projection; the USB-C port is for data transfer only.)
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Changing an existing boot disk to VirtIO
You may be able to change an existing VM without reinstalling, but the guest must have a usable driver for the new storage controller before it tries to boot from that disk. There is no universal migration sequence: steps vary by operating system, image, firmware, and QEMU configuration. Follow instructions for the specific guest version, and keep a recoverable copy of the VM or disk before changing boot-device configuration.
- Check the guest first. Confirm that the exact OS image has a working VirtIO storage driver; do not infer readiness solely from the operating system family.
- Consult version-specific migration guidance. The required driver preparation and boot changes differ by guest OS and VM setup.
- Preserve a recovery path. Retain a recoverable copy before changing the controller or boot configuration.
If Windows or Linux stops booting after a switch, missing or unavailable storage-controller drivers are a likely explanation, but not the only possible one. Diagnose the specific OS, image, firmware, and QEMU configuration rather than assuming a single cause. Restore the prior working configuration or recover from the copy if needed, then check guest-specific driver and boot instructions.
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Keep the bus separate from the disk format
The bus is the virtual device model presented to the guest; the disk format describes how disk data is stored. Raw and qcow2 are formats, not alternatives to virtio-blk or IDE. QEMU describes raw as simple and portable, and qcow2 as a versatile format that supports features including sparse images, compression, and snapshots. Changing a bus does not convert a disk image. See QEMU’s disk image documentation.
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A quick decision checklist
- Use
virtio-blkfor an ordinary disk when the guest has its driver. - Use a supported emulated controller, such as AHCI SATA for a modern x86 guest, when VirtIO is not ready.
- Use
virtio-scsifor CD-ROMs, full SCSI support, passthrough, or a large disk configuration. - Replace old
-hdaexamples with explicit device and backend configuration when building modern tooling. - Choose raw or qcow2 independently of the guest-facing bus.
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