Instance boot mode

Introduction to boot modes

The boot mode determines which mode is used to start the system disk when an ECS instance is started for the first time. The boot mode configured when the instance is created is inherited when the instance is restarted. ECS instances support two boot modes: BIOS and UEFI.

• BIOS mode: BIOS is the basic software layer in the system startup process. It is responsible for initializing hardware and providing basic hardware services to support the startup of the operating system. BIOS is a traditional firmware interface standard with limited functionality.

• UEFI mode: UEFI is a more advanced and modular firmware interface standard that provides a more powerful, flexible, and secure boot environment. UEFI mode has several advantages over BIOS mode, as described in the following section.

  Advantages of UEFI over BIOS

  • Support for large disk sizes

    Traditional BIOS supports only the Legacy boot mode, which relies on the MBR partition table and supports only disks smaller than 2 TB. UEFI uses the GUID Partition Table (GPT) and supports disks larger than 2 TB.

  • High speed and performance

    UEFI uses a parallel initialization mechanism, which typically provides faster startup speeds compared with the sequential hardware detection used by BIOS.

  • Security protection

    UEFI provides the Secure Boot feature. UEFI allows only authentic drivers and services to load at boot time and ensures that instances do not load malware on startup. UEFI requires digital signatures for drivers and the kernel. You can use UEFI to protect against piracy and boot-sector malware.

  • Extensibility

    UEFI is an extensible and standardized firmware interface, whereas BIOS is fixed, poorly documented, and entirely based on experience and obscure conventions as a de facto standard.

  • Usage notes

    UEFI is a 32-bit or 64-bit program that is written in the C language. UEFI is not limited to real mode and provides more addressable address space. BIOS is a 16-bit program assembly language that can run only in 16-bit real mode and provides only 1 MB of addressable memory.

Boot mode mechanism of ECS instances

When you create an ECS instance in the console, the boot mode of the selected instance type constrains the available image versions. For example, if you select a security-enhanced instance type that supports only the Unified Extensible Firmware Interface (UEFI) boot mode to create ECS instances, you can select only a UEFI image. When you create an instance by calling an API operation, you need to understand the factors that affect the boot mode of an ECS instance and select an appropriate combination to ensure that the instance loads the system disk in the expected mode.

The boot mode of an ECS instance is determined by both the instance type and the image boot mode. The instance boot mode is typically the boot mode that is supported by both the instance type and the image. If both support the UEFI boot mode, the instance preferentially uses the UEFI mode when it starts.

• The boot mode of an instance type can be UEFI, BIOS, or both UEFI and BIOS as defined by the system. The boot mode cannot be modified.

• The boot mode of an image can be UEFI, UEFI-Preferred, or BIOS.

  Note

  UEFI-Preferred indicates that the image supports both UEFI mode and BIOS mode.

  • Public image: The boot mode is defined by the system and cannot be modified.

  • Custom image: You can configure the boot mode of a custom image when you import or create the custom image.

View the boot mode of an instance type

You can query the boot mode of an instance type only by calling an API operation. You can view the boot mode in the SupportedBootModes parameter value returned by the DescribeInstanceTypes operation.

Boot mode of images

References

If you enable the image check feature when you create or import a custom image, the image check feature checks the boot mode of the image. For more information, see Overview of image check.