MBR vs. GPT Partition

Booting-in...

What a partition is?

Computer contains at least one hard drive (HDD) for data storage, but the physical drive can't store data directly before partitions are created. Strictly speaking, partition can't store data if files system is not built. Partition should be formatted to NTFS or FAT files system before storing data. One physical hard drive can be divided into several partitions (it is OK to have one partition only). How to create multiple partitions? This is when partition table come to use. Partition table defines and stores partition information, locating certain disk space on the beginning area. Operating system gains partition information by way of accessing data in partition table.

There are essentially two basic partition schemes: MBR & GPT.

MBR stands for Master Boot Recorder. 

GPT stands for GUID Partition Table. 

And GUID=Global Unique Identifier. 

What is? and Elucidation on MBR:

The MBR partitioning can have up to 4 partitions. But many users are not satisfied with 4 partitions and want to create more partitions. To eliminate this limitation, extended partition is introduced. Users can create multiples logical partitions in an extended partition. The following picture shows MBR partition scheme:

Main features of MBR partition table:

1. There are 4 primary partitions at most on MBR disk. The 64 bytes in the first sector is used to save partition information, and each partition requires 16 bytes.

2. There are three kinds of partitions on MBR disk: primary partition, extended partition and logical partition. Extended partition and logical partition are introduced for the purpose of breaking through the limit that MBR can have 4 primary partitions at most.

3. MBR partition table saves information about primary partition and extended partition only. Extended partition cannot store data directly, and logical partition should be created on extended partition. Logical partition information is stored in extended partition instead of partition table. Thus, users are able to create more than 4 partitions on MBR HDD.

4. Partition information stored in 16-byte space includes partition active mark, file system identifier, beginning and ending cylinder number, head number, sector number, start sector location (4 bytes), partition total sector (4 bytes), etc.

5. In general, the capacity of each sector is 512 bytes and the largest capacity it can express is 2TB. Thus, MBR partition can't start from disk space beyond 2TB and the maximum partition size can't exceed 2TB. Therefore, MBR partition scheme is not a proper solution for hard drive larger than 2TB.

If you are not a skilled user and not interested in technical details, you just need to remember the following aspects:

1. MBR partition scheme allows 4 primary partitions and this limitation is broken by extended partition and logical partition.

2. In MBR disk, there are 4 bytes to indicate address parameters, thus the starting location can't larger than 2TB and partition size can't exceed 2TB.

Some hard disk manufacturer attempts to extend sector size to break the restriction that MBR partition plan is not appropriate to disks larger than 2TB. That's why you can find some hard disks whose sector might be 1024 bytes or even 2048 bytes. However, extending sector brings problems such as disk speed. Thus, it is not widely accepted.

What is? and Elucidation on GPT:

GPT means GUID (Global Unique Identifier) partition table. Compared with MBR, GPT is relatively new partitioning scheme, gradually replacing MBR. 

From the picture above, we can draw following conclusions:

1. Reserve MBR. On GPT disk, the first physical sector on is still MBR which has talked in previous part. This MBR here does not have actual function and it exists only due to the consideration of software compatibility.

2. Primary GPT header stores right behind protective MBR on second physical sector. The GPT header defines partition number. Basically, you can deem that there is no limitation on partition number of GPT disk.

3. In fact, the third sector is the beginning of the actual partition table. Please note that each sector can store 4 partitions' information, which means every partition needs 128 bytes to store partition information.

4. We can calculate that each partition' information occupies a quarter of a sector, which is 128 bytes. In contrast, the partition information is 16 bytes in MBR disk, and thus GPT partitioning method has enough space to store beginning location, total capacity and other data. There is no need to worry about partition number or capacity.

5. On GPT disk all partitions' information is stored on partition table. However, in fact, if your computer is running Windows, GPT partition table usually occupies 32 sectors which are able to save information for 128 partitions. The rest space is reserved. In general, there won't be that many partitions on our computer, thus most space in partition table is not used.

6. The following two parts are partition table and backup for partition table header.

So much about GPT scheme and it is not that complicated. As to average users, the basic thing about GPT they should keep in mind is that it supports hard drives lager than 2TB and it is the major advantage over MBR.

Till now you may feel that you know how and when to use MBR and GPT: if hard drive is less than 2TB, both MBR and GPT are feasible; if hard drive's capacity exceeds 2TB, GPT is more appropriate. That's true, and yet the actual situation is more complex. To correctly make use of MBR and GPT, you need to know some additional knowledge about BIOS and EFI.

Preparing to hibernate....

Cheers.....