The layout of a disk with the GUID Partition Table. In this example, each logical block is 512 bytes in size and each entry has 128 bytes. The corresponding partition entries are assumed to be located in LBA2–33. Negative LBA addresses indicate a position from the end of the volume, with −1 being the last addressable block.
All modern personal computer operating systems support GPT. Some, including macOS and Microsoft Windows on the x86 architecture, support booting from GPT partitions only on systems with EFI firmware, but FreeBSD and most Linux distributions can boot from GPT partitions on systems with either the BIOS or the EFI firmware interface.
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 32bits. For hard disks with 512‑byte sectors, the MBR partition table entries allow a maximum size of 2TiB (2³²× 512‑bytes) or 2.20TB (2.20× 10¹²bytes).[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 64bits for logical block addresses, allowing a maximum disk size of 264 sectors. For disks with 512‑byte sectors, the maximum size is 8ZiB (264× 512‑bytes) or 9.44ZB (9.44× 10²¹bytes).[1] For disks with 4,096‑byte sectors the maximum size is 64ZiB (264× 4,096‑bytes) or 75.6ZB (75.6× 10²¹bytes).
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 4KB logical sectors within operating systems differs among their types, vendors and versions.[7] For example, Microsoft Windows supports 4Knative drives since Windows 8 and Windows Server 2012 (both released in 2012) in UEFI.[8]
Features
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.
MBR variants
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 2TiB, assuming a disk with 512 bytes per sector (see 512e). It would result in 16TiB with 4KiB 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 2TiB 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]
Partition table header (LBA 1)
GPT header format
Offset
Length
Contents
0 (0x00)
8 bytes
Signature ("EFI PART", 45h 46h 49h 20h 50h 41h 52h 54h or 0x5452415020494645ULL[lower-alpha 1] on little-endian machines)
8 (0x08)
4 bytes
Revision number of header - 1.0 (00h 00h 01h 00h) for UEFI 2.10
12 (0x0C)
4 bytes
Header size in little endian (in bytes, usually 5Ch 00h 00h 00h or 92 bytes)
16 (0x10)
4 bytes
CRC32 of header (offset +0 to +0x5b) in little endian, with this field zeroed during calculation
20 (0x14)
4 bytes
Reserved; must be zero
24 (0x18)
8 bytes
Current LBA (location of this header copy)
32 (0x20)
8 bytes
Backup LBA (location of the other header copy)
40 (0x28)
8 bytes
First usable LBA for partitions (primary partition table last LBA + 1)
48 (0x30)
8 bytes
Last usable LBA (secondary partition table first LBA − 1)
Starting LBA of array of partition entries (usually 2 for compatibility)
80 (0x50)
4 bytes
Number of partition entries in array
84 (0x54)
4 bytes
Size of a single partition entry (usually 80h or 128)
88 (0x58)
4 bytes
CRC32 of partition entries array in little endian
92 (0x5C)
*
Reserved; must be zeroes for the rest of the block (420 bytes for a sector size of 512 bytes; but can be more with larger sector sizes)
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 16bytes 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 16bytes 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:
Partition attributes
Bit
Content
0
Platform required (required by the computer to function properly, OEM partition for example, disk partitioning utilities must preserve the partition as is)
1
EFI firmware should ignore the content of the partition and not try to read from it
2
Legacy BIOS bootable (equivalent to active flag (typically bit 7 set) at offset +0h in partition entries of the MBR partition table)[14]
"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.
↑ In a multi-disk setup, non-UEFI bootloader (boot drive) requires MBR-based partitioning, while a system drive can use GUID partitioning.
↑ The GUIDs in this table are written as per RFC 4122, i.e. big-endianbyte order, recognizable by the position of the version bits. For example, the GUID for an EFI System partition (C12A7328-F81F-11D2-BA4B-00A0C93EC93B), when serialized in GPT data structures (little-endian), corresponds to the hex sequence 28 73 2A C1 1F F8 D2 11 BA 4B 00 A0 C9 3E C9 3B. The first three blocks are byte-swapped to little-endian, the last is a byte array. See details in TN2166[11]
↑ The formation of this GUID does not follow the GUID definition; it is formed by using the ASCII codes for the string "Hah!IdontNeedEFI". Such formation of "GUID" value breaks down the guaranteed uniqueness of GUID.
1 2 Some computer manufacturers have their own GUIDs for partitions that are analogous to the EFI System Partition, but that hold boot loaders to launch manufacturer-specific recovery tools.[42]
1 2 Previously, Linux used the same GUID for the data partitions as Windows (Basic data partition: EBD0A0A2-B9E5-4433-87C0-68B6B72699C7). Linux never had a separate unique partition type GUID defined for its data partitions. This created problems when dual-booting Linux and Windows in UEFI-GPT setup. The new GUID (Linux filesystem data: 0FC63DAF-8483-4772-8E79-3D69D8477DE4) was defined jointly by GPT fdisk and GNU Parted developers.[44] It is identified as type code 0x8300 in GPT fdisk.
1 2 The GUID for /usr on Solaris is used as a generic GUID for ZFS by macOS.
1 2 NetBSD and MidnightBSD had used the FreeBSD GUIDs before their unique GUIDs were created.
↑ The Ceph filesystem uses GUIDs to mark the state of preparation a disk is in.[66][67]
↑ The legacy Fuchsia GUIDs had two oddities: UUIDs were not generated randomly (several runs of bits were common between partitions), and partitions were uniquely identified by type GUID. The standardized scheme uses randomly-generated GUIDs, and slotted partitions (e.g. zircon_{a,b,r}) share the same type and are distinguished by name and unique GUID.[82]
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↑ The GPT header contains a field that specifies the size of a partition table entry. The minimum required is 128 bytes, but implementations must allow for other values. See "Mac Developer Library". Developer.Apple.com. Apple. Retrieved 2014-07-13.
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.
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