GUID Partition Table ( GPT ) is a standard for partition table layout on physical storage devices used on desktop or PC servers, such as hard disk drives or solid-state drives, using a unique global identifier (GUID). Although this is part of the Unified Extensible Firmware Interface (UEFI) standard (Unified EFI Forum proposes replacement for the PC BIOS), it is also used on some BIOS systems due to the limited boot master partition (MBR) partition table, which uses 32 bits to store block addresses logical (LBA) and size information on the traditional 512-byte disk sector.
All modern PC operating systems support GPT. Some, including macOS and Microsoft Windows on x86, 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 legacy and EFI firmware firmware interfaces.
Video GUID Partition Table
History
The widespread MBR partition scheme, dating from the early 1980s, imposes restrictions that affect the use of modern hardware. One of the major limitations is the use of 32 bits to store block addresses and quantity information. For hard disks with 512-byte sectors, MBR partition table entries allow up to 2 TiB (2 32 Ã, ÃÆ'â € "512 bytes).
Therefore Intel developed a new partition table format in the late 1990s as part of what eventually became UEFI. In 2010, GPT formed a subset of UEFI specifications. GPT allocates 64 bits for logical block addresses, thus allowing maximum disk size of 2 64 sectors. For disks with 512-byte sectors, the maximum size is 9.4 ZB (9,4 ÃÆ'â € "10 21 bytes) or 8Ã, ZiB (9,444,732,965,739,290,427,392 bytes, originating from 18,446,744,073,709,551,616 (2 64 ) sector ÃÆ'â € "512 (2 9 ) bytes per sector).
Maps GUID Partition Table
Features
Like modern MBR, GPT uses logical block addressing (LBA) instead of the historical cylinder-head-sector (CHS) handling. The protective MBR is contained in LBA 0, the GPT header is in LBA 1, and the GPT header has a pointer to the partition table, or Partition Entry Array, usually LBA 2. The UEFI specification specifies that a minimum of 16,384 bytes, regardless of sector size, is allocated to the Partition Entry Array. On disks that have 512-byte sectors, an array partition size of 16,384 bytes and a minimum size of 128 bytes for each partition entry, LBA 34 is the first usable sector on the disk.
Hard-disk manufacturers make the transition to 4096-byte sectors. The first drive continues to present the 512-byte physical sector to the OS, so degraded performance can occur when the 4-KB hard-disk sector boundaries do not coincide with the 4 KB logical blocks, clusters and virtual memory pages that are common across multiple operating systems and systems files. This is a special problem in writing, when the drive is forced to perform two read-write operations to comply with a misaligned 4A writing operation.
For backwards compatibility with most older operating systems such as DOS, OS/2, and Windows versions prior to Vista, MBR partitions should always start at the track boundary according to traditional CHS addressing schemes and terminated on cylinder boundaries. This also applies to partitions with encapsulated CHS geometry (as reflected by the BIOS and CHS sector entries in the MBR partition table) or partitions accessed only via LBA. Additional partitions should start at the cylinder boundary as well. This usually causes the first primary partition starting in LBA 63 on disks accessed through LBA, leaving a gap of 62 sectors with MBR based disks, sometimes called "MBR slots", "boot paths", or "embedding areas". Unused disk space is usually used by bootloaders like GRUB to store the second stage.
MBR variant
MBR Protector (LBA 0)
For limited backward compatibility, legacy MBR space is still backed up in the GPT specification, but is now used in a way that prevents the MBR based disk utilities from recognizing and possibly overwriting GPT disks. This is referred to as Protective MBR .
One type of EEh partition, which includes the entire GPT drive (where "whole" actually means as many drives as can be represented in the MBR), is indicated and identifies it as GPT. Operating systems and tools that can not read GPT disks generally recognize the disk as containing an unknown type partition and no free space, and will usually refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental deletion. In addition, the GPT-aware OS can check the protective MBR and if the attached partition type is not EEh type or if there are multiple partitions defined on the target device, the OS can refuse to manipulate the partition table.
While MBR and MBR protector layouts are defined around 512 bytes per sector, the actual sector size can be larger on various media such as MO disk or hard disk with Advanced Format.
If the actual size of the disk exceeds the maximum partition size represented by using the legacy 32-bit LBA entry in the MBR partition table, the recorded size of this partition is truncated to the maximum, thus ignoring the remaining disk. This amount is for a maximum size of 2 TiB, assuming a disk with 512 bytes per sector (see 512e). This will generate 16 TiB with sector 4Ã, KiB (4Kn), but since many older operating systems and tools are hard wired for sector sizes 512 bytes or limited to 32-bit calculations, over limit of TiB 2 may cause compatibility issues.
Hybrid_MBR_.28LBA_0_.2B_GPT.29 "> MBR Hybrid (Hybrid_MBR_ (LBA_0 _ _ GPT)">
Partition table header (LBA 1)
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.
Partition entry (LBA 2-33)
After the header, the Partition Entry Array describes the partition, using a minimum size of 128 bytes for each entry block. The initial location of the array on the disk, and the size of each entry, is given in the GPT header. The first 16 bytes of each entry point to a unique global identifier of the partition type (GUID). For example, the GUID for an EFI system partition is C12A7328-F81F-11D2-BA4B-00A0C93EC93B . The second 16 bytes is a unique GUID for the partition. Then follow the start and end of the 64 bit LBAs, the partition attribute, and the Unicode 36 character part name (max.). As with the nature and purpose of the GUID, no central registry is required to ensure the uniqueness of the GUID partition type determiner.
The 64-bit partition table attribute is shared between 48-bit common attributes for all types of partitions, and special attributes of 16-bit types:
Microsoft defines custom attribute types for basic data partitions as:
Google specifies the custom-type attributes for the Chrome OS kernel as:
Operating system support
UNIX and Unix-like UNIX and UNIX-like
Windows: 32-bit version
Windows 7 and earlier do not support UEFI on 32-bit platforms, and therefore do not allow boot from GPT partitions.
Windows: 64-bit version
GUID partition type
See also
Note
References
External links
- Microsoft TechNet: Disk Sector on GPT Disk (archived page)
- Microsoft TechNet: Disk and File System Troubleshooting
- Microsoft TechNet: Using GPT Drives
- Microsoft: FAQ on GPT disk usage in Windows
- Microsoft Technet: How to Basic Disk and Volume Work Little MS-specific but good numbers related GPT to the older MBR format and MBR-protection, showing full disk layout, and how to interpret hexdumps tables.
- Apple Developer Connection: GPT Secret
- Maximize large drives with GPT and Linux
- Change boot Windows Vista SP1 or 7 x86_64 from BIOS-MBR mode to UEFI-GPT mode without Reinstall
- Support for GPT (Partitioning scheme) and HDD larger than 2.19 TB in Microsoft Windows XP
- Setting up RAID volume under Linux with & gt; 2TB disk
Source of the article : Wikipedia
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