Windows and GPT FAQ

The GUID Partition Table (GPT) was introduced as part of the Unified Extensible Firmware Interface (UEFI) initiative. GPT provides a more flexible mechanism for partitioning disks than the older Master Boot Record (MBR) partitioning scheme that was common to PCs.

A partition is a contiguous space of storage on a physical or logical disk that functions as if it were a physically separate disk. Partitions are visible to the system firmware and the installed operating systems. Access to a partition is controlled by the system firmware before the system boots the operating system, and then by the operating system after it is started.

MBR disks support only four partition table entries. If more partitions are wanted, a secondary structure known as an extended partition is necessary. Extended partitions can then be subdivided into one or more logical disks.

By convention, Windows creates MBR disk partitions and logical drives on cylinder boundaries based on the reported geometry, although this information no longer has any relationship to the physicalcharacteristics of the hardware (disk driver or RAID controller). Starting with Windows Vista and Windows Server 2008, more logical boundaries are selected when the hardware provides better hints at the true cache or physical alignment. Because this partition information is stored on the drive itself, the operating system is not dependent on the alignment.

MBR partitioning rules are complex and poorly specified. For example, does cylinder alignment mean that each partition must be at least one cylinder in length? An MBR partition is identified by a two-byte field, and coordination is necessary to avoid collision. IBM originally provided that coordination; today there is no single authoritative list of partition identifiers.

Another common practice is to use partitioned or "hidden" sectors to hold specific information. That practice is undocumented and results in severe system problems that are difficult to debug. Over the years, vendor-specific implementations and tools were released to the public, making support difficult.

GPT disks can grow to a very large size. The number of partitions on a GPT disk is not constrained by temporary schemes such as container partitions as defined by the MBR Extended Boot Record (EBR).

The GPT disk partition format is well defined and fully self-identifying. Data critical to platform operation is located in partitions and not in unpartitioned or "hidden" sectors. GPT disks use primary and backup partition tables for redundancy and CRC32 fields for improved partition data structure integrity. The GPT partition format uses version number and size fields for future expansion.

Each GPT partition has a unique identification GUID and a partition content type, so no coordination is necessary to prevent partition identifier collision. Each GPT partition has a 36-character Unicode name. This means that any software can present a human-readable name for the partition without any additional understanding of the partition.

Basic disks are the storage types most often used with Windows. The term basic disk refers to a disk that contains partitions, such as primary partitions and logical drives, and these in turn are usually formatted with a file system to become a volume for file storage.

The GPT format for a basic disk is illustrated in the following figure.

Figure 1. GPT Format for Basic Disks

The protective MBR area exists on a GPT partition table for backward compatibility with disk management utilities that operate on MBR. The GPT header defines the range of logical block addresses that are usable by partition entries. The GPT header also defines its location on the disk, its GUID, and a 32-bit cyclic redundancy check (CRC32) checksum that is used to verify the integrity of the GPT header. Each entry in the GUID partition table begins with a partition type GUID. The 16-byte partition type GUID, which is similar to a System ID in the partition table of an MBR disk, identifies the type of data that the partition contains and identifies how the partition is used, for example, whether it is a basic disk or a dynamic disk. Note that each GUID partition entry has a backup copy.

For more information about basic disks, refer to the MSDN topic "Basic and Dynamic Disks."

Dynamic disks were first introduced with Windows 2000 and provide features that basic disks do not, such as the ability to create volumes that span multiple disks (spanned and striped volumes) and the ability to create fault-tolerant volumes (mirrored and RAID-5 volumes). Like basic disks, dynamic disks can use the MBR or GPT partition styles on systems that support both.

For more information about dynamic disks, refer to the MSDN topic "Basic and Dynamic Disks."

In theory, a GPT disk can be up to 2^64 logical blocks in length. Logical blocks are commonly 512 bytes in size.

The maximum partition (and disk) size is a function of the operating system version. Windows XP and the original release of Windows Server 2003 have a limit of 2TB per physical disk, including all partitions. For Windows Server 2003 SP1, Windows XP x64 edition, and later versions, the maximum raw partition of 18 exabytes can be supported. (Windows file systems currently are limited to 256 terabytes each.)

If this occurred, you must have used an MBR-only-aware disk tool to access the GPT disk. For more information, see the following questions:

• Can a disk be both GPT and MBR?
• What is a Protective MBR?
• Why does the GPT have a Protective MBR?

GPT and MBR disks can be mixed on systems that support GPT, as described earlier. However, you must be aware of the following restrictions:

• Systems that support UEFI require that boot partition must reside on a GPT disk. Other hard disks can be either MBR or GPT.
• Both MBR and GPT disks can be present in a single dynamic disk group. Volume sets can span both MBR and GPT disks.

Removable media without either GPT or MBR formatting is considered a "superfloppy." The entire media is treated as a single partition.

The media manufacturer performs any MBR partitioning of removable media. If the media does have an MBR, only one partition is supported. There is little user-discernible difference between MBR-partitioned media and superfloppies.

Examples of removable media include floppy disk drives, JAZ disk cartridges, magneto-optical media, DVD-ROM, and CD-ROM. Hard disk drives on external buses such as SCSI or IEEE 1394 are not considered removable.

Detachable disks are typically expected to migrate between computers or simply to be unavailable to the operating system at times. Examples of detachable disks are USB disks, which can be easily disconnected by the end-user.

Windows XP supports only MBR partitioning on detachable disks. Later versions of Windows support GPT partitions on detachable disks.

For more about removable media, see the following questions:

• What about removable media?
• What is a superfloppy?

The ESP contains the NTLDR, HAL, Boot.txt, and other files that are needed to boot the system, such as drivers. The Partition GUID defines the ESP:

The ESP should only include files that are required for booting an operating system, platform tools that run before operating system boot, or files that must be accessed before operating system boot. For example, files that are required for performing pre-boot system maintenance must be placed in the ESP.

Other value-add files or diagnostics used while the operating system is running should not be placed in the ESP. It is important to note that the space in the ESP is a limited system resource; its primary purpose is to provide storage for the files that are needed to boot the operating system.

The preferred option is for system manufacturers to place value-add contents in an OEM-specific partition. Just like MBR OEM partitions, the contents of GPT OEM (or other unrecognized) partitions are not exposed (given drive letters or returned in volume lists). Users are warned that deleting the partition can cause the system to fail to operate. An OEM-specific partition should be placed before the MSR and after any ESP on the disk. Although not architectural, this placement has the same benefits as placing the ESP first. For example, it is also impossible to span volumes when an OEM-specific partition is logically between the two data partitions that you are attempting to span.

Placement in the ESP is an option for applications or files that execute in the pre-operating system boot environment. However, the ESP is architecturally shared space and represents a limited resource. Consuming space in the ESP should be considered carefully. Files that are not relevant to the pre-operating system boot environment should not be placed in the ESP.

The Microsoft Reserved Partition (MSR) reserves space on each disk drive for subsequent use by operating system software. GPT disks do not allow hidden sectors. Software components that formerly used hidden sectors now allocate portions of the MSR for component-specific partitions. For example, converting a basic disk to a dynamic disk causes the MSR on that disk to be reduced in size and a newly created partition holds the dynamic disk database. The MSR has the Partition GUID:

When initially created, the size of the MSR depends on the size of the disk drive:

• On drives less than 16GB in size, the MSR is 32MB
• On drives greater than or equal two 16GB, the MSR is 128 MB.

As the MSR is divided into other partitions, it becomes smaller.

For UEFI systems, the boot drive must contain an ESP, an MSR, and at least one basic data partition that contains the operating system. Only one ESP should exist on a system even if multiple operating systems are installed on that system. In a mirrored boot configuration there may actually be two drives with an ESP but they are considered to be a redundant copy of the same ESP. Each data drive must contain at least an MSR and one basic data partition.

All basic data partitions on the drive should be contiguous. As noted above, placing an OEM-specific or other unrecognized partition between data partitions imposes limitations on later volume spanning.

It has the following partition type GUID:

Windows exposes only basic data partitions. Other partitions with FAT file systems may be mounted, but not exposed only programmatically. Only basic data partitions are assigned drive letters or mount points.

The ESP FAT file system is mounted, but not exposed. This allows programs running under Windows to update the contents of the ESP. Assigning a drive letter to the ESP using ‘mountvol /s’ will allow access to the partition. Access to the ESP requires admin privilege.

Although the MSR, and any partitions created from the MSR, could have recognizable file systems, none are exposed.

Any OEM-specific partitions or partitions associated with other operating systems are not recognized by Windows. Unrecognized partitions with recognizable file systems are treated like the ESP. They will be mounted, but not exposed. Unlike MBR disks, there is no practical difference between OEM-specific partitions and other operating system partitions; all are "unrecognized."

Dynamic disks use two different GPT partitions:

• A data container partition that corresponds to the MBR partition 0x42, with the following GUID:

  DEFINE_GUID (PARTITION_LDM_DATA_GUID, 0xAF9B60A0L, 0x1431, 0x4F62, 0xBC,
  0x68, 0x33, 0x11, 0x71, 0x4A, 0x69, 0xAD);

• A partition to contain the dynamic configuration database, with the following GUID:

  DEFINE_GUID(PARTITION_LDM_METADATA_GUID, 0x5808C8AAL, 0x7E8F, 0x42E0, 0x85,
  0xD2, 0xE1, 0xE9, 0x04, 0x34, 0xCF, 0xB3);

• Volumes are created in the data container and mounted by default. Again, this is exactly the same as the contents of 0x42 MBR partitions.

For a drive to be eligible for conversion to dynamic, all basic data partitions on the drive must be contiguous. If other unrecognized partitions separate basic data partitions, the disk cannot be converted. This is one of the reasons that the MSR must be created before any basic data partitions.

The first step in conversion is to separate a portion of the MSR to create the configuration database partition. All non-bootable basic partitions are then combined into a single data container partition. Boot partitions are retained as separate data container partitions. This is analogous to conversion of primary partitions.

Windows XP and later versions of the Windows operating system differs from Windows 2000 in that basic and extended partitions are preferentially converted to a single 0x42 partition, rather than being retained as multiple distinct 0x42 partitions as on Windows 2000.

You can access the GPT disk partitions of different types using the tools that are listed in the following table.

By using the Microsoft Platform SDK APIs, you can also develop your own tools to access the GPT disk partitions at their primitive levels.

You can convert an existing partition format to another format. For more information, see the following TechNet articles:

• Change a Master Boot Record Disk into a GUID Partition Table Disk
• Change a GUID Partition Table Disk into a Master Boot Record Disk

Yes. However, there are some key caveats. The OEM Preinstallation Kit (OPK) initializes the Disk and Partition GUIDs to zero. On first boot of Windows, the operating system generates unique GUIDs. The OPK only supports generation of ESP, MSR, and basic data partitions.

If an application has recorded any Disk or Partition GUIDs it may break. Any applications, drivers, utilities, or firmware implementations supplied by system manufacturers or application vendors that rely on GUIDs should be capable of handling GUIDs that change from the OPK initialization values to those generated by the operating system.

It is a way for OEMs to simplify operating system preinstallation and system recovery. This command can easily be extended to create a "default" disk configuration for the platform. For example, the system manufacturer could extend the MAKE command to automatically partition the boot drive with an ESP, MSR, an OEM-specific partition, and one basic data partition.

For example, consider a possible disk configuration called BOOT_DISK. In the event of business failure recovery, MAKE BOOT_DISK would allow the customer to completely repartition a boot disk to the original factory defaults.