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GUID Partition Table

System Administration GUID Partition Table Note

Introduction

  • MBR and GPT are partition schemes
    • GPT is a standard for the layout of the partition table on a physical hard disk, using globally unique identifiers (GUID).
  • UEFI <-> GPT, BIOS <-> MBR

Master Boot Record (MBR)

Structure

The Master Boot Record (MBR) is the first 512 bytes of a storage device. It contains:

  • An operating system bootloader - 446 bytes
  • The storage device’s partition table (4 primary partitions) - 64 bytes
  • MBR boot signature (0xAA55) - 2 bytes

Constraints

  • 4 primary partition or 3 primary + 1 extended partitions
    • Arbitrary number of logical partitions within the extended partition
  • The logical partition meta-data is stored in a linked-list structure.
  • One byte partition type codes which leads to many collisions
  • Maximum addressable size is 2 TiB, i.e. any space beyond 2 TiB cannot be defined as a partition
    • MBR stores partition sector information using 32-bit LBA values
    • 512 bytes per sector
    • 512 bytes * (2^32) = 2 TiB

Booting Process

  • System initialization with firmware called BIOS
  • The BIOS looks for the bootloader on the MBR of the first storage device or the first partition of the device, then executes it
  • Bootloader reads the partition table
    • Conventional Windows/DOS MBR bootlaoder will check the partition table for one and only one active and primary partition
    • GRUB safely ignores this

  • Loading Operating system

  • Common GNU/Linux bootloader include GRUB and Syslinux

Unified Extensible Firmware Interface (UEFI)

  • To replace BIOS
  • Provides legecy support for BIOS
  • The original EFI specification was developed by Intel
  • The UEFI specfication is managed by the Unified EFI forum
  • Micro operating system
  • Graphical User Interface
  • Secure Computing (evil)

GUID Partition Table (GPT)

  • GUID stands for Globally Unique Identifier
  • Part of the UEFI specification
  • Solves some legacy problems with MBR but also may have compatibility issues.
  • Can be used also on BIOS system via a protective MBR

Advantages

  • Up to 18 EB (1 EB = 1024 TB) ?
  • No partition type collision because of GUIDs
  • 8 ZiB
    • GPT uses 64-bit LBA
    • 512 bytes per sector
    • 512 bytes * (2^64) = 8 ZiB

Tools

  • fdisk
  • gdisk
  • parted
  • gpart