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. Although it is a part of the Extensible Firmware Interface (EFI) standard (Intel's proposed replacement for the PC BIOS), it is widely used on BIOS systems because of the limitations of MBR partition tables, which restrict maximum disk size to 2 TiB.

Diagram illustrating the layout of the GUID Partition Table scheme. In this example, each logical block (LBA) is 512 bytes in size, and each partition entry is 128 bytes. LBA addresses that are negative indicate position from the end of the volume, with −1 being the last addressable block.

Features

Current MBR-based partition table schemes insert the partitioning information in the master boot record (MBR) itself (which on BIOS system is also the container for code that begins the process of initializing the disk). In GPT, partition table information is stored in the GPT header, but to maintain 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.

Like modern MBRs, GPT uses logical block addressing (LBA) in place of the historical cylinder-head-sector (CHS) addressing of legacy MBRs. 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,^[1] "Do not assume that the block size is always going to be 512 bytes." Modern storage devices such as solid-state drives may contain 1024-byte LBAs. Drives which boot Intel-based Macs are typically formatted with a GUID Partition Table, rather than with the Apple Partition Map.

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

Legacy MBR (LBA 0)

The MBR at the beginning of the disk was originally designed to prevent MBR-based disk utilities from mis-recognizing, and possibly over-writing, GPT disks (however, in Operating Systems that support GPT-based boot, it is nowadays also used to store the first stage of the bootloader). A single partition type of 0xEE, encompassing the entire GPT drive, is indicated and identifies it as GPT. 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 will typically refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental erasures. Furthermore, GPT-aware OSes will check the protective MBR and if the enclosed partition type is not of type 0xEE or if there are multiple partitions defined on the target device, the device should not be manipulated.

If the disk is larger than 2 TiB, the maximum partition size representable using the 32-bit LBA's of the legacy MBR (assuming a 512 byte block size), the size of this partition is marked as 2 TiB, ignoring the rest of disk.

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 EFI specification requires that a minimum of 16,384 bytes be reserved for the partition table, so this gives rise to 128 partition records.)

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 may be verified by the firmware, bootloader and/or Operating System on boot. Because of this, hex editors should not be used to modify the contents of the GPT. Such modification would render the checksum invalid. In this case, the primary GPT may be overwritten with the secondary one, or, if both GPTs contained invalid checksums, would be unable to access the disk.

The values for current and backup LBAs of the primary header should be the second sector of the disk (1) and the last sector of the disk, respectively. The secondary header at the end of the disk identifies its own table of partition entries, which is located directly before that header.

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. As is the nature and purpose of GUIDs, no central registry is needed to ensure the uniqueness of the GUID partition type designators.

According to Apple, "Do not hardwire the current size of the partition entry (128 bytes)." Microsoft TechNet says that attributes are divided into two halves: the lower 4 bytes representing partition independent attributes, and the upper 4 bytes are partition type dependent. Microsoft uses the following bits in general:^{[clarification needed]}

OS support of GPT

Hybrid MBRs are non-standard and can be interpreted in different ways by different OSes. ^[8] Unless otherwise noted, OSes provide precedence to the GPT data when a hybrid MBR configuration is encountered.

Partition type GUIDs

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. ^a b  Linux and Windows use the same GUID for their respective data partitions.
3. ^a b  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

• Master boot record
• GUID
• EFI
• Disk partitioning

References

1. ^ Technical Note TN2166: Secrets of the GPT
2. ^ "Ubuntu on MacBook"
3. ^ "GPT fdisk"
4. ^ "rEFIt - Myths and Facts About Intel Macs"
5. ^ "Windows and GPT FAQ"
6. ^ "Windows and GPT FAQ"
7. ^ Hybrid MBRs: The Good, the Bad, and the So Ugly You'll Tear Your Eyes Out
8. ^ Hybrid MBRs: The Good, the Bad, and the So Ugly You'll Tear Your Eyes Out

External links

• Microsoft TechNet: Disk Sectors on GPT Disks
• Microsoft TechNet: Using GPT Drives on x86-64 Systems
• Apple Developer Connection: Secrets of the GPT
• Make the most of large drives with GPT and Linux
• GPT fdisk : Information on Hybrid GPT-MBR, Converting MBR and BSD disklabels to GPT and Booting from GPT disks
• Microsoft : FAQs on Using GPT disks in Windows