# GUID Partition Table (简体中文)

相关文章

- Arch boot process
- Master Boot Record
- Unified Extensible Firmware Interface
- Partitioning

GUID Partition Table (GPT) is a new style of partitioning which is part of the Unified Extensible Firmware Interface Specification, using the globally unique identifier for devices. It is different from the Master Boot Record (the more commonly used partitioning style) in many aspects and has many advantages.

To understand GPT, it is important to understand what is MBR and what are its disadvantages.

For any partitioning style, the number of partitions that can be defined is based on the total space allotted for the partition table and the space required for storing the information of a single partition.

## Master Boot Record

存储在硬盘的第一个扇区的MBR分区表分区信息如下

HDD的位置 | 作用
--- | ---
前 440 个字节 | 由BIOS锁存的MBR启动代码
441-446 字节 | MBR盘标识
447-510 字节 | 实际的主分区和扩展分区的分区表信息 。(注，此处未列出逻辑分区)
511-512 字节 | MBR 启动标识 0xAA55。

有64字节的空间用来描述主分区的信息。为了扩展它，启用了扩展分区的概念。 An extended partition is simply a primary partition in the MBR which acts like a container for other partitions called logical partitions. So one is limited to either 4 primary partitions, or 3 primary and 1 extended partitions with many logical partitions inside it.

### MBR 带来的问题

1. 一块磁盘只能存在 4 个主分区或者是最多 3 个主分区加上 1 个扩展分区（扩展分区里面可以建立任意数量的逻辑分区）。 如果你已经有 3 个主分区加上一个扩展分区，就算硬盘上还存在一些自由空间，你也不能在上面新建任何的分区。
2. 扩展分区中的逻辑分区的元数据被存储在一个链表结构。如果一条链路丢失，所有的逻辑分区后的元数据都将丢失。
3. MBR只支持1字节的分区类型代码，从而导致一些冲突。
4. MBR分区扇区存储信息使用32位LBA值。LBA 使用扇区为 512 比特（较常用）导致 2 TiB 为的磁盘的最大寻址。如果使用 MBR 超过 2 TiB 的磁盘便不能被识别或者是分区。

## GUID Partition Table（GPT 分区表）

GUID 分区表（GPT）使用 GUID 分区定义及其类型（Linux中叫做 UUID ），因此得名。GPT 分区表包括：

硬盘上的位置 | 作用
--- | ---
磁盘的第一逻辑扇区或前 512 字节 | 保护MBR-与MBR相像，只是64字节区域包含一个单个0XEE类型的主分区入口，这个入口支持>2TiB的空间，分区大小高达2TiB
磁盘的第二个逻辑扇区或下一个 512 直接 | Primary GPT Header - Contains the Unique Disk GUID, Location of the Primary Partition Table, Number of possible entries in partition table, CRC32 checksums of itself and the Primary Partition Table, Location of the Secondary (or Backup) GPT Header
16 KiB (by default) following the second logical sector of the disk | Primary GPT Table - 128 Partition entries (by default, can be higher), each with an entry of size 128 bytes (hence total of 16 KiB for 128 partition entries). Sector numbers are stored as 64-bit LBA and each partition has a Partition Type GUID and a Unique Partition GUID.
16 KiB (by default) before the last logical sector of the disk | Secondary GPT table - It is byte-for-byte identical to the Primary table. Used mainly for recovery in case the primary partition table is damaged.
最后一个逻辑扇区或最后 512 字节 | Secondary GPT Header - Contains the Unique Disk GUID, Location of the Secondary Partition Table, Number of possible entries in the partition table, CRC32 checksums of itself and the Secondary Partition Table, Location of the Primary GPT Header. This header can be used to recover GPT info in case the primary header is corrupted.

### Advantages of GPT

1. Uses GUIDs (UUIDs) to identify partition types - No collisions.
2. Provides a unique disk GUID and unique partition GUID for each partition - A good filesystem-independent way of referencing partitions and disks.
3. Arbitary number of partitions - depends on space allocated for the partition table (maximum of 128 partitions) - No need for extended and logical partitions.
4. Uses 64-bit LBA for storing Sector numbers - maximum addressable disk size is 2 ZiB.
5. Stores a backup header and partition table at the end of the disk that aids in recovery in case the primary ones are damaged.
6. CRC32 checksums to detect errors and corruption of the header and partition table.

### Kernel Support

`CONFIG_EFI_PARTITION` option in the kernel config enables GPT support in the kernel (despite the name EFI PARTITION). This options must be built-in the kernel and not compiled as a loadable module. This option is required even if GPT disks are used only for data storage and not for booting. This option is enabled by default in Arch's linux and linux-lts kernels in [core] repo. In case of a custom kernel enable this option by doing `CONFIG_EFI_PARTITION=y`.

## 引导支持

### UEFI 系统

自从 GPT 成为 UEFI 规范的一部分且强制其为UEFI的引导,所有 UEFI 引导程序都支持GPT磁盘。 阅读 UEFI_Bootloaders 可以获得更多的信息。

### BIOS 系统

- GRUB2 需要一个2 MiB 的“BIOS引导分区” （在 gdisk 中分区代码为 EF02，在 GNU Parted 中使用 bios_grub 标志）。in BIOS systems to embed its `core.img` file due to lack of post-MBR embed gap in GPT disks. Runtime GPT support in GRUB2 is provided by the `part_gpt` module. See GRUB2#GPT specific instructions for more info.

- Syslinux requires the `/boot` partition to be marked as "Legacy BIOS Bootable" GPT attribute (legacy_boot flag in GNU Parted) to identify the partition containing the syslinux boot files by its MBR boot code `gptmbr.bin` . See Syslinux#GUID_Partition_Table_aka_GPT for more info.

- GRUB Legacy, present in the AUR as grub-legacy, does not support GPT disks. Fedora's heavily patched GRUB Legacy fork grub-legacy-fedora-git contains GPT patches from Intel (tested in Fedora, not tested in Arch).

Note: Fedora developers have mentioned that after the release of Fedora 17, grub-legacy-fedora development will stop. Fedora already uses GRUB2 as its default BIOS bootloader since F16. Users are recommended to switch to GRUB2 or Syslinux instead.

- LILO's GPT support has not been tested so it is unclear whether it has issues booting in GPT disks.

## Partitioning Utilities

### GPT fdisk

GPT fdisk is a set of text-mode utilities for editing GPT disks. It consists of gdisk, sgdisk and cgdisk which are equivalent to respective tools from util-linux fdisk (used for MBR disks). It is available in the [extra] repository as gptfdisk.

Note: The fdisk partitioning utilities from util-linux (i.e. fdisk, cfdisk and sfdisk) do not support GPT, and may damage the GPT header and partition table if used on a GPT disk.

#### Convert from MBR to GPT

One of the best features of gdisk (and sgdisk and cgdisk too) is its ability to convert MBR and BSD disklabels to GPT without data loss. Upon conversion, all the MBR primary partitions and the logical partitions become GPT partitions with the correct partition type GUIDs and Unique partition GUIDs created for each partition.

Just open the MBR disk using gdisk and exit with "w" option to write the changes back to the disk (similar to fdisk) to convert the MBR disk to GPT. Watch out for any error and fix them before writing any change to disk because you may risk losing data. See http://www.rodsbooks.com/gdisk/mbr2gpt.html for more info. After conversion, the bootloaders will need to be reinstalled to configure them to boot from GPT.

Note: Remember that GPT stores a secondary table at the end of disk. You must make sure that the last 1 MiB of the disk is not used by any partition.
Note: Keep in mind that if your Boot-Manager is GRUB, it needs a BIOS Boot Partition. If your MBR Partitioning Layout isn't too old, there is a good chance that the first partition starts at sector 2048 for alignment reasons. That means at the beginning will be 1007 KiB of empty space where this bios-boot partition can be created. To do this, first do the mbr->gpt conversion with gdisk as described above. Afterwards, create a new partition with gdisk and manually specify its position to be sectors 34 - 2047, and set the `EF02` partition type.

### GNU Parted

In GNU Parted >=3.0, the `parted` command-line utility does not support any filesystem related operation, and most of the FS related code has been removed from the libparted, leaving only minimal code required by external applications like gparted. The upstream recommends using the filesystem specific tools or one of the parted's GUI wrappers like gparted (which calls these external tools) for filesystem related operations.

## See also

1. Wikipedia's Page on GPT and MBR
2. Homepage of Rod Smith's GPT fdisk tool and its Sourceforge.net Project page - gptfdisk
3. Rod Smith's page on Converting MBR to GPT and Booting OSes from GPT
4. Rod Smith's page on the New Partition Type GUID for Linux data partitions
5. System Rescue CD's page on GPT
6. Wikipedia page on BIOS Boot Partition
7. Make the most of large drives with GPT and Linux - IBM Developer Works
8. Microsoft's Windows and GPT FAQ

Category:

- File systems (简体中文)