Monday, June 27, 2016

ESXi boot mode - UEFI or BIOS

Legacy BIOS bootstrapping along with a master boot record (MBR) is uses with x86 compatible systems for ages. The concept of MBRs was publicly introduced in 1983 with PC DOS 2.0. It is unbelievable that we are still using the same concept after more then 30 years.

However, there must be some limitations in 30 years old technology, isn't it?

BIOS limitations (such as 16-bit processor mode, 1 MB addressable space and PC AT hardware) had become too restrictive for the larger server platforms. The effort to address these concerns began in 1998 and was initially called Intel Boot Initiative later renamed to EFI. In July 2005, Intel ceased its development of the EFI specification at version 1.10, and contributed it to the Unified EFI Forum, which has evolved the specification as the Unified Extensible Firmware Interface (UEFI).

What is EFI (or UEFI) firmware?

UEFI replaces the the old Basic Input/Output System (BIOS). UEFI can be used on

  • physical server booting ESXi hypervisor or 
  • Virtual Machine running on top of ESXi hypervisor. 
This blog post is about ESXi boot mode however just for completeness I would like to mention that VMware Virtual Machine with at least hardware version 7 supports UEFI as well. For further information about VM UEFI look at [1].

Originally called Extensible Firmware Interface (EFI), the more recent specification is known as Unified Extensible Firmware Interface (UEFI), and the two names are used interchangeably.

EFI (Extensible Firmware Interface) is a specification for a new generation of system firmware. An implementation of EFI, stored in ROM or Flash RAM, provides the first instructions used by the CPU to initialize hardware and pass control to an operating system or bootloader. It is intended as an extensible successor to the PC BIOS, which has been extended and enhanced in a relatively unstructured way since its introduction. The EFI specification is portable, and implementations may be capable of running on platforms other than PCs.
For more information, see  the Wikipedia page for Unified Extensible Firmware Interface.

General UEFI Advantages

UEFI firmware provides several technical advantages over a traditional BIOS system:

  • Ability to boot from large disks (over 2 TB) with a GUID Partition Table (GPT)
  • CPU-independent architecture
  • CPU-independent drivers
  • Flexible pre-OS environment, including network capability
  • Modular design
  • Since UEFI is platform independent, it may be able to enhance the boot time and speed of the computer. This is especially the case when large hard drives are in use. 
  • UEFI can perform better while initializing the hardware devices.
  • UEFI can work alongside BIOS. It can sit on top of BIOS and work independently.
  • It supports MBR and GPT partition types.

Note: Modern systems are only emulating the legacy BIOS. They are EFI native.


vSphere 5.0 and above supports booting ESXi hosts from the Unified Extensible Firmware Interface (UEFI). With UEFI, you can boot systems from hard drives, CD-ROM drives, USB media, or network.

UEFI benefits

  • ESXi can boot from a disk larger than 2 TB provided that the system firmware and the firmware on any add-in card that you are using supports it. 

UEFI drawbacks

  • Provisioning with VMware Auto Deploy requires the legacy BIOS firmware and is not available with UEFI BIOS configurations. I hope that this limitation will be lifted soon.

Notes: Changing the host boot type between legacy BIOS and UEFI is not supported after you install ESXi 6.0. Changing the boot type from legacy BIOS to UEFI after you install ESXi 6.0 might cause the host to fail to boot.


UEFI is meant to completely replace BIOS in the future and bring in many new features and enhancements that can’t be implemented through BIOS. BIOS can be used in servers that do not require large storage for boot. To be honest, even you can, it is not very common to use boot disks greater then 2 TB for ESXi hosts therefore you may be using BIOS at the moment, but I would recommend shifting to UEFI, as it is the future while BIOS will fade away slowly.

ESXi hypervisor supports both boot modes therefore if you have modern server hardware and don't use VMware Auto Deploy then UEFI should be your preferred choice.

[1] VMware : Using EFI/UEFI firmware in a VMware Virtual Machine 
[2] VMware : Best practices for installing ESXi 5.0 (VMware KB 2005099)
[3] VMware : Best practices to install or upgrade to VMware ESXi 6.0 (VMware KB 2109712)
[4] Usman Khurshid : [MTE Explains] Differences Between UEFI and BIOS
[5] Wikipedia : Unified Extensible Firmware Interface

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