Understanding GPT(GNU Partition Table) and MBR in Linux.

Today we will check what is MBR and GPT, what are their differences/similarities and what should we use ?

1. What is MBR ?
MBR(master boot record) is the first boot sector on our hard drive i.e. at the very beginning of partitioned computer mass storage devices like hard drives i.e. fixed disks or removable drives.

However the MBR based partitioning mechanism is now superseded by the GUID Partition Table (GPT) mechanism in new computer systems.
A GPT partitonaing scheme can coexist with the MBR based scheme in order to provide some backward compatibility with old systems.

2. MBR & GPT Standards Description
Master Boot Record(MBR) and GUID Partition Table (GPT) are two different standard to define the layout of the partition table on a physical storage device which is used on desktop and server systems. Both mechanisms use the concept of globally unique identifiers (GUID) or UUID(Universally unique identifiers) to mark each disk/drive as unique.

GPT is a part of the Unified Extensible Firmware Interface (UEFI) standard.

3. MBR VS GPT Layout.
MBR stand for Master Boot Record is now an old standard to manage the partitions in a hard disk.
GPT stands for GUID Partition Table is another disk type and it is the latest disk layout standard defining partition table of a disk.

MBR(512 bytes) is made up of three parts, which consists executable code called the Master Boot Code(446 bytes), the partition table(16*4=64bytes) for the disk and the disk signature(or checksum of 2 bytes).

To understand the differences between these two schemes, lets have a look at below table.

Bootstrap Code Area 446 Bytes Protective MBR(can contain MBR data)
Partition entry 1 16Bytes
Partition entry 2 16Bytes
Partition entry 3 16Bytes
Partition entry 4 16Bytes
Boot Signature 2 Bytes
Partition 1 GUID Partition Table Header
Partition 2 GUID Partition Entry 1
Partition 3 GUID Partition Entry 2
Extended Partition GUID Partition Entry …n..128
Logical Drive 1 Partition 1
Logical Drive 2 Partition 2
Logical Drive 3 Partition n
Logical Drive 4 Partition 128
Logical Drive 5 GUID Partition Entry 1
Logical Drive 6 GUID Partition Entry 2
Logical Drive n GUID Partition Entry …n..128
Logical Drive 128 GUID Partition Table Header Backup


4. GPT vs MBR Differences
a. Primary Partitions
MBR can have 4 primary partitions at max. However GPT can have 128 primary partitions.

b. Disk Capacity
MBR can only support 2TB hard drives at max. GPT can support 18EB disk sizes even.

c. Backup Partition Table: Added advantage of GPT
GPT saves a copy of backup partition table which can be recovered when primary got corrupted.
Even in terms of security, it can verify the table using checksum and correct it from backup.

d. OS & Hardware Support
MBR supports all OSes and hardwares.
However GPT support is sometimes limited to UEFI based hardware.


5. What should you use GPT or MBR ?
It all depends on the requirement and hardware support.
We prefer to use GPT on large SSDs if we need to make more primary partitions on the disk and if our hardware motherboard support UEFI boot.

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