GUID Partition Table


GUID partition table (GPT) is a latest type of partitioning which is a division of UEFI (Unified Extensible Firmware Interface) specification. Also from various aspects it is totally different from a Master Boot Record and has many advantages. Some of them are listed below.

Advantages of GPT:

  • To identify partition types with no collisions it uses GUIDs (UUIDs)
  • Maximum addressable size of disk is 2 ZIB and uses LBA of 64-bit for storing sector numbers
  • Grants a distinct disk GUID and distinct partition GUID for every partition - A superior file system-independent mode of mentioning partitions and disks
  • In case of primary header corruption and damaged partition table, it stores a copy of then at the last part of the disk that aids in revival
  • To detect any errors or corruption of either header or partition table, CRC32 checksums are used

In order to understand what GPT is, first it’s important to know what MBR is and its disadvantages. The MBR (Master Boot Record) partition table stores the partition info in the initial sector of hard disk.The complete information with reference to the primary partitions is restricted to the 64 bytes allotted. In order to extend this we use extended partitions where extended partition is just a primary partition in the MBR which operates similar to a container of other partitions called logical partitions. Hence it is restricted to either 4 primary partitions or 3 primary partitions plus 1 extended partitions with several logical partitions within it.

Problems Faced with MBR:

  •  Merely 3 primary plus 1 extended partition / 4 primary partitions can be created. If you got a little open space outside an extended partition area and 3 primary plus 1 extended partitions, then its not possible to create new partitions above that free space
  • MBR supports partition type codes of merely 1 byte, which results in many collisions
  • The logical partition’s meta-data is usually stored in linked-list structure inside the extended partition. If you lose one link every part of the logical partitions subsequent to that metadata are lost
  • If MBR partition is used then any space beyond 2TiBthen it cannot be defined as a partition.

This was all regarding MBR, now lets move on to GPT. Go through the following details to know what GPT consists of

  • HDD location: Primary logical sector of the disk / First 512 bytes
  • Function: Protective MBR – It is similar to normal MBR although the 64-byte area enclose a solitary 0xEE type Primary partition entry defined on entire size of the disk or if >2 TiB, till 2 Tib partition size.

  • HDD location: Secondary logical sector of the disk / Second 512 bytes
  • Function: Primary GPT Header – It holds the Unique Disk GUID along with location of the Primary Partition Table, Number of feasible partition table entries, CRC32 checksums, the Primary Partition Table and Secondary GPT Header’s location.

  • HDD location: 16 KiB (default) follows the secondary logical sector of the disk
  • Function: Primary GPT Table - 128 Partition entries with an entry of size 128 bytes each. Each partition has a Partition of type GUID and a distinct Partition GUID and sector numbers are stored as 64-bit LBA.

  • HDD location: 16 KiB (default) preceded by final logical sector of the disk
  • Function: Secondary GPT table - It is byte-for-byte matching to the Primary table and is used mainly for restoring primary partition table that is corrupted

  • HDD location: Final logical sector of the disk or Last 512 bytes
  • Function: Secondary GPT Header - Holds a Unique Disk GUID along with Secondary Partition table’s location, Number of entries possible in the partition table, CRC32 checksums and the Secondary Partition Table, Location of Primary GPT Header. If in case the primary header is corrupted then this header can be used to improve GPT info.

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