GPT Disk

GUID Partition Table

In computer hardware, GUID Partition Table (GPT) is a standard for the layout of the partition table on a physical hard disk. It is a part of the Extensible Firmware Interface (EFI) standard proposed by Intel as a replacement for the PC BIOS, one of the few remaining relics of the original IBM PC. EFI uses GPT whereas BIOS uses a Master Boot Record (MBR).


Current PC BIOS schemes use a Master Boot Record (MBR) to begin the process of initializing the disk. The MBR begins with an entry called the Master Boot Code, which contains an executable binary for the purpose of identifying and booting the active partition. PC BIOS's replacement, EFI, itself contains this capability but to maintain backwards compatibility, GPT retains the MBR entry as the first sector on the disk followed by a Primary Partition Table Header, the actual beginning of GPT.

GPT uses modern logical block addressing (LBA) in place of the cylinder-head-sector (CHS) addressing used with MBR. Legacy MBR information is contained in LBA 0, the GPT header is in LBA 1, and the partition table itself follows. In 64-bit Windows operating systems, 16,384 bytes, or 32 sectors, are reserved for the GPT, leaving LBA 34 as the first usable sector on the disk.

According to Apple, "Do not assume that the {LBA} size is always going to be 512 bytes."

GPT also provides for redundancy. The GPT header and partition table are written at both the beginning and end of the disk.

Legacy MBR (LBA 0)

The primary purpose of the MBR at the beginning of the disk is to prevent MBR-based disk utilities from mis-recognizing, and possibly over-writing, GPT disks. A single partition, encompassing the entire GPT drive, is indicated. The System ID for the partition is set to 0xEE, indicating that it uses GPT. Because of this, EFI ignores the MBR. Some 32-bit OSes which cannot read GPT disks nevertheless recognize this ID and present the disk as an inaccessible GPT disk. Older OSes will generally recognize the disk as containing one partition of unknown type and no empty space, and then they'll typically also refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This way, accidental erasures are minimized.

Partition table header (LBA 1)

The partition table header defines the blocks on the disk that can be utilized by the user (the usable blocks). It also defines the number and size of the partition entries that make up the partition table. On 64-bit Windows Server 2003 machines, there are 128 partition entries reserved, each 128 bytes long. Thus, 128 partitions can be created.

The header contains the disk GUID (Globally Unique Identifier). It records its own size and location (always LBA 1) and the size and location of the secondary GPT header and table (always the last sectors on the disk). Importantly, it also contains a CRC32 checksum for itself and for the partition table, which is verified by EFI processes on boot. Because EFI uses and verifies this checksum, hex editors should not be used to modify the contents of the GPT. Such modification would render the checksum invalid. In this case, EFI would overwrite the primary GPT with the secondary one, or, if both GPTs contained invalid checksums, would be unable to access the disk.

Partition table format
Offset Length Contents
0 8 bytes Signature ("EFI PART", 45 46 49 20 50 41 52 54)
8 4 bytes Revision (For version 1.0, the value is 00 00 01 00)
12 4 bytes Header size (in bytes, usually 5C 00 00 00 meaning 92 bytes)
16 4 bytes CRC32 of header (cyclic redundancy check)
20 4 bytes reserved, must be zero
24 8 bytes Primary LBA (this LBA, always 1)
32 8 bytes Backup LBA (always equal to the last LBA on the disk)
40 8 bytes First usable LBA for partitions (last partition table LBA + 1)
48 8 bytes Last usable LBA (backup LBA - partition table size - 1)
56 16 bytes Disk GUID (also referred as UUID on UNIXes)
72 8 bytes Partition entries starting LBA (always right after header, 2)
80 4 bytes Number of partition entries
84 4 bytes Size of a partition entry (usually 128)
88 4 bytes CRC32 of partition array
92 * reserved, must be zeroes for the rest of the block (420 bytes for a 512-byte LBA)

Partition entries (LBA 2–33)

Partition entries are simple and straightforward. The first 16 bytes designate the partition type GUID. For example, the GUID for an EFI System partition is {C12A7328-F81F-11D2-BA4B-00A0C93EC93B}. The second 16 bytes contain a GUID unique to the partition. Starting and ending 64-bit LBAs are also recorded here, and space is allocated for partition names and attributes. Unfortunately, there is no central registry for GUID partition type designators, either de jure or de facto.

GUID partition entry format
Offset Length Contents
0 16 bytes Partition type GUID
16 16 bytes Unique partition GUID
32 8 bytes First LBA (little-endian)
40 8 bytes Last LBA (inclusive, usually odd)
48 8 bytes Attribute flags (e.g. bit 60 denotes read-only)
56 72 bytes Partition name (36 UTF-16LE code units)
According to Apple, "Do not hardwire the current size of the partition entry (128 bytes)."

Partition type GUIDs

Assoc. OS Partition type Globally-Unique Identifier (GUID)
(None) Unused entry 00000000-0000-0000-0000-000000000000
MBR partition scheme 024DEE41-33E7-11D3-9D69-0008C781F39F
EFI System Partition C12A7328-F81F-11D2-BA4B-00A0C93EC93B
BIOS Boot Partition 21686148-6449-6E6F-744E-656564454649
Windows Microsoft Reserved Partition E3C9E316-0B5C-4DB8-817D-F92DF00215AE
Basic Data Partition EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
Logical Disk Manager metadata partition 5808C8AA-7E8F-42E0-85D2-E1E90434CFB3
Logical Disk Manager data partition AF9B60A0-1431-4F62-BC68-3311714A69AD
HP-UX Data partition 75894C1E-3AEB-11D3-B7C1-7B03A0000000
Service Partition E2A1E728-32E3-11D6-A682-7B03A0000000
Linux Data partition EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
RAID partition A19D880F-05FC-4D3B-A006-743F0F84911E
Swap partition 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F
Logical Volume Manager (LVM) partition E6D6D379-F507-44C2-A23C-238F2A3DF928
Reserved 8DA63339-0007-60C0-C436-083AC8230908
FreeBSD Boot partition 83BD6B9D-7F41-11DC-BE0B-001560B84F0F
Data partition 516E7CB4-6ECF-11D6-8FF8-00022D09712B
Swap partition 516E7CB5-6ECF-11D6-8FF8-00022D09712B
Unix File System (UFS) partition 516E7CB6-6ECF-11D6-8FF8-00022D09712B
Vinum volume manager partition 516E7CB8-6ECF-11D6-8FF8-00022D09712B
ZFS partition 516E7CBA-6ECF-11D6-8FF8-00022D09712B
Mac OS X Hierarchical File System (HFS+) partition 48465300-0000-11AA-AA11-00306543ECAC
Apple UFS 55465300-0000-11AA-AA11-00306543ECAC
ZFS 6A898CC3-1DD2-11B2-99A6-080020736631
Apple RAID partition 52414944-0000-11AA-AA11-00306543ECAC
Apple RAID partition, offline 52414944-5F4F-11AA-AA11-00306543ECAC
Apple Boot partition 426F6F74-0000-11AA-AA11-00306543ECAC
Apple Label 4C616265-6C00-11AA-AA11-00306543ECAC
Apple TV Recovery partition 5265636F-7665-11AA-AA11-00306543ECAC
Solaris Boot partition 6A82CB45-1DD2-11B2-99A6-080020736631
Root partition 6A85CF4D-1DD2-11B2-99A6-080020736631
Swap partition 6A87C46F-1DD2-11B2-99A6-080020736631
Backup partition 6A8B642B-1DD2-11B2-99A6-080020736631
/usr partition 6A898CC3-1DD2-11B2-99A6-080020736631
/var partition 6A8EF2E9-1DD2-11B2-99A6-080020736631
/home partition 6A90BA39-1DD2-11B2-99A6-080020736631
EFI_ALTSCTR 6A9283A5-1DD2-11B2-99A6-080020736631
Reserved partition 6A945A3B-1DD2-11B2-99A6-080020736631
NetBSD Swap partition 49F48D32-B10E-11DC-B99B-0019D1879648
FFS partition 49F48D5A-B10E-11DC-B99B-0019D1879648
LFS partition 49F48D82-B10E-11DC-B99B-0019D1879648
RAID partition 49F48DAA-B10E-11DC-B99B-0019D1879648
concatenated partition 2DB519C4-B10F-11DC-B99B-0019D1879648
encrypted partition 2DB519EC-B10F-11DC-B99B-0019D1879648

  1. The GUIDs in this table are written assuming a little-endian byte order. For example, the GUID for an EFI System partition is written as C12A7328-F81F-11D2-BA4B-00A0C93EC93B here, which corresponds to the 16 byte sequence 28 73 2A C1 1F F8 D2 11 BA 4B 00 A0 C9 3E C9 3B — only the first three blocks are byte-swapped.
  2. Linux and Windows use the same GUID for their respective data partitions.
  3. The GUID for /usr on Solaris is used as a generic GUID for ZFS by Mac OS X.
  4. Definitions are in src/sys/sys/disklabel_gpt.h. NetBSD had used the FreeBSD GUIDs before unique NetBSD-specific GUIDs were created.


See also

External links

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