What is a disk partition table?
What is a Disk Partition Table?
Disk Partition Table is a concept frequently encountered in system operations and maintenance. The selection and pre-configuration of a disk partition table often determine the system installation method, boot mode, and partition planning approach. So, what exactly is a disk partition table? Today, I’ll break it down for you in detail.
The Essence of a Disk Partition Table
Is a disk partition table a physical entity, a piece of code, or something intangible?
In fact, a disk partition table is a string of binary numbers stored on each disk. It is saved in the first few sectors of every disk.
Classification of Disk Partition Tables
Disk partition tables fall into two categories: MBR partition table and GPT partition table.
GPT partition table is also known as the GUID partition table.
MBR Partition Table
First, before introducing the MBR partition table, one concept needs clarification: the three letters “MBR” can also refer to MBR boot code.
Similarly, MBR boot code is binary data located in the head sectors of a disk, and it is contiguous with the MBR partition table. Together, they occupy 512 bytes at the beginning of the disk.
The MBR partition table is an older partition table format, suitable for older computers with Legacy BIOS firmware.
- Location
The MBR boot code + MBR partition table starts at the first sector of the hard disk (logical sector number 0), with a fixed total size of 512 bytes. - Partition Table: Immediately following the boot code, it occupies 64 bytes, containing 4 partition table entries (16 bytes each).
- End Marker: The last 2 bytes are fixed as
0x55AA, used to verify the validity of the sector.
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As you can see, since the partition table occupies only 64 bytes and contains just 4 partition table entries, a disk using the MBR partition table can have at most 4 primary partitions. However, you can create more partitions by designating one primary partition as an extended partition and then creating multiple logical partitions within it.
Since the partition size field is a 32-bit sector count, with a default sector size of 512 bytes, the maximum size for a single partition is:
2^32^ × 512 Bytes ≈ 2 TiB
The entire disk is also subject to this limitation. Disks larger than 2 TiB cannot use the MBR partition table and must switch to GPT.
Additionally, an MBR disk has only one MBR partition table with no backup. If this MBR partition table becomes corrupted, all partitions on the disk may become unreadable.
In contrast, GPT partition table disks have a redundant backup, making the partition table more secure and adding an extra layer of data protection.
- Key Points:
- Older Legacy BIOS machines can only use disks with the MBR partition table.
- Disks larger than 2 TB cannot be recognized.
GPT (GUID) Partition Table
In contrast, the GPT partition table is a more modern and advanced partition table format.
The GPT partition table exists not only in the head sectors of the disk but also has an identical copy in the very last sectors of the disk, serving as a backup of the head partition table. If the head partition table is accidentally damaged, the tail partition table can still be read.
The GPT partition table records the partition type GUID, the partition’s unique GUID, the disk sector range occupied by the partition, partition attributes (hidden, read-only, bootable, etc.), and the partition name.
- Supports Large Capacity: 64-bit addressing, capable of recognizing disks up to several ZB in size.
- Large Number of Partitions: Supports up to 128 partitions, far exceeding MBR’s 4-primary-partition limit.
- Multiple Backups and Checksums: Full table CRC checksum greatly improves reliability.
Please note!
Legacy BIOS firmware cannot recognize GPT partition table disks; it can only use MBR partition table disks as system boot disks.
Newer UEFI firmware can recognize both MBR and GPT partition table disks, and both types can be used to install and boot the system!
For UEFI firmware computers, as long as there is an EFI partition (ESP partition), i.e., a FAT partition, booting can be completed, regardless of the partition table type!