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Overview

A GUID partition table (GPT) is a standard for arranging partition tables on physical computer storage devices, such as hard drives or SSDs, which universally unique identifiers (UUIDs) uses (also known as GUIDs) to clearly distinguish partitions on a storage device. GPT is UEFI (Unified Extensible Firmware Interface) is part of the standard but is certain BIOSsystems, because the system used in BIOS Master Boot Record (MBR) its partition tables are limited because they use 32 bits for logical block addressing (LBA) of traditional 512-byte disk sectors.

All modern personal computer operating systems support GPT. However, some systems, including x86-based macOS and Microsoft Windows, only support booting from GPT partitions on computers running EFI / UEFI (these systems can also handle hard disks formatted with a GPT partition table in the BIOS). , they just can't boot from them), but FreeBSD and most Linux distributions can also boot from the GPT partition, regardless of whether the machine is running the old BIOS or modern EFI / UEFI.

history

The Master Boot Record (MBR) partitioning scheme, which has been widely used since the early 1980s, has placed limitations on the use of modern hardware. Its biggest limitation was addressing a block of only 32 bits, which resulted in a maximum of 512TB (2TB) of MBR partition tables on hard disks with 2-byte sectors.³² x 512 bytes) of storage, so they couldn't access the area above that.

In the late 1990s, Intel developed a new partition table format as part of what eventually became the Unified Extensible Firmware Interface (UEFI). As of 2010, the GUID partition table is part of the UEFI specification. GPT uses 64-bit Boolean block addressing, which allows 2-bit processing⁶⁴ sector. This is a maximum size of 512 for a 9,4-byte sector storage ZB (9,4 x 10²¹ byte) or 8 ZiB (2⁶⁴ piece sector x 512 bytes per sector).

features

Like MBRs, GPTs use logical block addressing (LBA) instead of previous cylinder-head-sector (CHS) addressing. The protective MBR is located in block 0 of the LBA (see below). The GPT header is in block 1 of the LBA, which contains a pointer to the partition table (Partition Entry Array), which usually begins with Unit 2 of the LBA. The UEFI specification requires at least 16,384 bytes to be reserved for the Partition Entry Array, regardless of the size of the sectors, with entries of 128 bytes. Thus, on a disk with 512-byte sectors, sector 34 is the first usable sector on the disk.

Hard drive manufacturers have switched to sector sizes of 4096 bytes for their new products. The first such drives continued to display 512-byte physical sectors in operating systems. Performance degradation can occur if the physical 4-KiB sector boundaries of a drive do not coincide with the 4 KiB logical blocks, clusters, and virtual memory pages that are common to many operating systems and file systems. This was a particular problem during write operations where the drive was forced to perform two read-modify-write operations to perform a single misaligned 4 KiB write operation.

Protective MBR

For limited backward compatibility, the old MBR is still retained in the GPT specification, but is now used only to prevent MBR-based disk management software from being recognized incorrectly and possibly overwriting GPT disks. This is called a protective MBR.

It contains a single 0xEE (GPT) type partition that, according to the protective MBR record, occupies the entire disk as a single partition. Operating systems and devices that cannot read GPT disks usually mark the disk as unknown, with no free space on it, and usually refuse to modify the disk (unless the user specifically requests and confirms, for example, deleting the partition). This minimizes accidental deletions. In addition, operating systems that know GPT can check the protective MBR, and if the included partition type is not type 0xEE, or if multiple partitions are defined on the drive, the operating system can also refuse to modify the partition table.

If the actual size of the disk exceeds the maximum partition size that can be represented by inherited 32-bit LBA entries in the MBR partition table, the fixed size of the partition must be truncated to the maximum, thus ignoring the rest of the disk. This means a maximum size of 2 TiB, assuming a disk sector size of 512 bytes. This would mean 16 TiB if the sector size were set to 4 KiB according to the newer standard, but since many older operating systems and utilities are fixed to a sector size of 512 bytes or limited to 32-bit computations, exceeding the 2 TiB size limit is a compatibility. can cause problems.