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How To Fix QEMU Warning: “Image format was not specified for ‘flash0.img’ and probing guessed raw”

Ubuntu 14

In this article, we will discuss a common warning encountered when using QEMU (Quick Emulator) to emulate ARM architecture. The warning message is as follows: “Image format was not specified for ‘flash0.img’ and probing guessed raw”. This warning occurs when QEMU is unable to determine the disk format for raw devices created using the dd command. We’ll walk you through the process of resolving this warning by explicitly specifying the disk format as raw.

Quick Answer

To fix the QEMU warning "Image format was not specified for ‘flash0.img’ and probing guessed raw," you can explicitly specify the disk format as raw in your QEMU command. This can be done by adding format=raw to the -drive options for the relevant disk images.

Understanding the Warning

Before we delve into the solution, it’s important to understand what the warning message means. When you use QEMU to emulate an ARM system, it needs to know the format of the disk images it’s working with. In this case, it’s unable to determine the format for flash0.img and flash1.img, so it makes an educated guess that the format is raw.

This warning doesn’t necessarily mean something is wrong, but it does indicate that QEMU is making assumptions about your system that may not be accurate. This could potentially lead to problems down the line, so it’s best to address it.

The Solution

To resolve this warning, you can modify your QEMU command to explicitly specify the disk format as raw. Here’s an example of how to do this:

sudo qemu-system-arm -m 1024 -cpu cortex-a57 -M virt -nographic -drive file=flash0.img,format=raw,if=pflash -drive file=flash1.img,format=raw,if=pflash -drive if=none,file=xenial-server-cloudimg-arm64-uefi1.img,id=hd0 -device virtio-blk-device,drive=hd0 -device virtio-net-device,netdev=net0,mac=$randmac -netdev type=tap,id=net0

In the command above, we’ve added format=raw to the -drive options for flash0.img and flash1.img. This explicitly tells QEMU that the disk format is raw, eliminating the need for it to guess.

Understanding the Command Parameters

Let’s break down the command parameters to better understand what they do:

  • -m 1024: This specifies the amount of memory for the guest system. In this case, it’s set to 1024 MB.
  • -cpu cortex-a57: This sets the CPU model.
  • -M virt: This tells QEMU to use the virt machine type, which is a versatile platform designed for emulating ARM systems.
  • -nographic: This disables graphical output so that the system is purely command-line based.
  • -drive file=flash0.img,format=raw,if=pflash: This tells QEMU to use flash0.img as a drive with a raw format. The if=pflash part specifies that this is a parallel flash device.
  • -drive file=flash1.img,format=raw,if=pflash: This does the same as the previous parameter, but for flash1.img.
  • -drive if=none,file=xenial-server-cloudimg-arm64-uefi1.img,id=hd0: This sets up a hard drive with the specified image file.
  • -device virtio-blk-device,drive=hd0: This connects the previously defined hard drive to the system.
  • -device virtio-net-device,netdev=net0,mac=$randmac -netdev type=tap,id=net0: These parameters set up a network device.

By understanding these parameters, you can better troubleshoot any issues that may arise when using QEMU.


In conclusion, the QEMU warning “Image format was not specified for ‘flash0.img’ and probing guessed raw” can be easily fixed by explicitly specifying the disk format as raw in your command. This not only resolves the warning but also ensures that QEMU correctly interprets your disk images, preventing potential issues down the line.

How do I check the disk format of a file?

You can use the file command followed by the path to the file. For example, file flash0.img will display the format of the flash0.img file.

Can I use a different disk format instead of raw?

Yes, you can use different disk formats such as qcow2 or vmdk. Simply replace raw with the desired format in the -drive options of your QEMU command.

What are the possible disk formats supported by QEMU?

QEMU supports various disk formats including raw, qcow2, vmdk, vdi, and more. You can check the QEMU documentation for a complete list of supported formats.

Will explicitly specifying the disk format affect the performance of QEMU?

Specifying the disk format will not significantly impact the performance of QEMU. The main purpose is to ensure that QEMU correctly interprets the disk image. The performance of QEMU is more influenced by factors such as CPU model, memory allocation, and other configuration settings.

Can I use the `-drive` option multiple times in the QEMU command?

Yes, you can use the -drive option multiple times to specify multiple disk images or devices in your QEMU command. Just make sure to provide the appropriate file path, format, and interface (if) for each drive.

What is the purpose of the `-nographic` option in the QEMU command?

The -nographic option disables graphical output and redirects the console output to the terminal. This is useful when you want to run QEMU in a purely command-line environment without any graphical interface.

How can I troubleshoot other issues related to QEMU?

If you encounter other issues with QEMU, it’s recommended to refer to the QEMU documentation, search online forums and communities, or ask specific questions on platforms like Stack Overflow. Additionally, checking the QEMU command parameters and ensuring correct file paths and formats can help in troubleshooting.

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