GUID Partition Table

The Master Boot Record (MBR) partitioning scheme, widely used since the early 1980s, imposed limitations for use of modern hardware. The available size for block addresses and related information is limited to 32 bits. For hard disks with 512‑byte sectors, the MBR partition table entries allow a maximum size of 2 TiB (2³² × 512‑bytes) or 2.20 TB (2.20 × 10¹² bytes).^[1]

In the late 1990s, Intel developed a new partition table format as part of what eventually became the Unified Extensible Firmware Interface (UEFI). The GUID Partition Table is specified in chapter 5 of the UEFI 2.8 specification.^[2] GPT uses 64 bits for logical block addresses, allowing a maximum disk size of 2⁶⁴ sectors. For disks with 512‑byte sectors, the maximum size is 8 ZiB (2⁶⁴ × 512‑bytes) or 9.44 ZB (9.44 × 10²¹ bytes).^[1] For disks with 4,096‑byte sectors the maximum size is 64 ZiB (2⁶⁴ × 4,096‑bytes) or 75.6 ZB (75.6 × 10²¹ bytes).

In 2010, hard-disk manufacturers introduced drives with 4,096‑byte sectors (Advanced Format).^[3] For compatibility with legacy hardware and software, those drives include an emulation technology (512e) that presents 512‑byte sectors to the entity accessing the hard drive, despite their underlying 4,096‑byte physical sectors.^[4] Performance could be degraded on write operations, when the drive is forced to perform two read-modify-write operations to satisfy a single misaligned 4,096‑byte write operation.^[4] Since April 2014, enterprise-class drives without emulation technology (4K native) have been available on the market.^[5][6]

Readiness of the support for 4 KB logical sectors within operating systems differs among their types, vendors and versions.^[7] For example, Microsoft Windows supports 4K native drives since Windows 8 and Windows Server 2012 (both released in 2012) in UEFI.^[8]

Like MBR, GPT uses logical block addressing (LBA) in place of the historical cylinder-head-sector (CHS) addressing. The protective MBR is stored at LBA 0, and the GPT header is in LBA 1, with a backup GPT header stored at the final LBA. The GPT header has a pointer to the partition table (Partition Entry Array), which is typically at LBA 2. Each entry on the partition table has a size of 128 bytes.The UEFI specification stipulates that a minimum of 16,384 bytes, regardless of sector size, are allocated for the Partition Entry Array.^[9] Thus, on a disk with 512-byte sectors, at least 32 sectors are used for the Partition Entry Array, and the first usable block is at LBA 34 or higher, while on a 4,096-byte sectors disk, at least 4 sectors are used for the Partition Entry Array, and the first usable block is at LBA 6 or higher.

Protective MBR (LBA 0)

For limited backward compatibility, the space of the legacy Master Boot Record (MBR) is still reserved in the GPT specification, but it is now used in a way that prevents MBR-based disk utilities from misrecognizing and possibly overwriting GPT disks. This is referred to as a protective MBR.^[10]

A single partition of type EEh, encompassing the entire GPT drive (where "entire" actually means as much of the drive as can be represented in an MBR), is indicated and identifies it as GPT. Operating systems and tools which cannot read GPT disks will generally recognize the disk as containing one partition of unknown type and no empty space, and will typically refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental erasures.^[10] Furthermore, GPT-aware OSes may check the protective MBR and if the enclosed partition type is not of type EEh or if there are multiple partitions defined on the target device, the OS may refuse to manipulate the partition table.^[11]

If the actual size of the disk exceeds the maximum partition size representable using the legacy 32-bit LBA entries in the MBR partition table, the recorded size of this partition is clipped at the maximum, thereby ignoring the rest of the disk. This amounts to a maximum reported size of 2 TiB, assuming a disk with 512 bytes per sector (see 512e). It would result in 16 TiB with 4 KiB sectors (4Kn), but since many older operating systems and tools are hard coded for a sector size of 512 bytes or are limited to 32-bit calculations, exceeding the 2 TiB limit could cause compatibility problems.^[10]

Hybrid MBR (LBA 0 + GPT)

In operating systems that support GPT-based boot through BIOS services rather than EFI, the first sector may also still be used to store the first stage of the bootloader code, but modified to recognize GPT partitions. The bootloader in the MBR must not assume a sector size of 512 bytes.^[10]

The partition table header defines the usable blocks on the disk. It also defines the number and size of the partition entries that make up the partition table (offsets 80 and 84 in the table).^[2]: 119

After the primary header and before the backup header, the Partition Entry Array describes partitions, using a minimum size of 128 bytes for each entry block.^[12] The starting location of the array on disk, and the size of each entry, are given in the GPT header. The first 16 bytes of each entry designate the partition type's globally unique identifier (GUID). For example, the GUID for an EFI system partition is C12A7328-F81F-11D2-BA4B-00A0C93EC93B. The second 16 bytes are a GUID unique to the partition. Then follow the starting and ending 64 bit LBAs, partition attributes, and the 36 character (max.) Unicode partition name. As is the nature and purpose of GUIDs and as per RFC 4122, no central registry is needed to ensure the uniqueness of the GUID partition type designators.^{[13][2]: 2200}

The 64-bit partition table attributes are shared between 48-bit common attributes for all partition types, and 16-bit type-specific attributes:

Microsoft defines the type-specific attributes for basic data partition as:^[15][16]

Google defines the type-specific attributes for ChromeOS kernel as:^[17]

UNIX and Unix-like systems

Windows: 32-bit versions

Windows 7 and earlier do not support UEFI on 32-bit platforms, and therefore do not allow booting from GPT partitions.^[31]

Windows: 64-bit versions

Limited to 128 partitions per disk.^[31]

"Partition type GUID" means that each partition type is strictly identified by a GUID number unique to that type, and therefore partitions of the same type will all have the same "partition type GUID". Each partition also has a "partition unique GUID" as a separate entry, which as the name implies is a unique id for each partition.

• Microsoft TechNet: Disk Sectors on GPT Disks (archived page)
• Microsoft Windows Deployment: Converting MBR to GPT without dats loss
• Microsoft TechNet: Troubleshooting Disks and File Systems
• Microsoft TechNet: Using GPT Drives
• Microsoft: FAQs on Using GPT disks in Windows
• Microsoft Technet: How Basic Disks and Volumes Work A bit MS-specific but good figures relate GPT to older MBR format and protective-MBR, shows layouts of complete disks, and how to interpret partition-table hexdumps.
• Apple Developer Connection: Secrets of the GPT
• Make the most of large drives with GPT and Linux
• Convert Windows Vista SP1+ or 7 x86_64 boot from BIOS-MBR mode to UEFI-GPT mode without Reinstall
• Support for GPT (Partition scheme) and HDD greater than 2.19 TB in Microsoft Windows XP
• Setting up a RAID volume in Linux with >2TB disks