GUID Partition Table
In computer hardware, GPT stands for GUID Partition Table (GPT), and is a standard for the layout of the partition table on a physical hard disk. It forms a part of the Extensible Firmware Interface (EFI) standard, which is Intel's proposed replacement for the PC BIOS. It is also used on some BIOS systems because of the limitations of MBR partition tables. GPT allows for a maximum disk and partition size of 9.4 zettabytes (9.4 × 1021 bytes[1]) As of 2010[update], most current operating systems support GPT, although some operating systems (including Mac OS X and Windows) require systems with EFI hardware to support booting from GPT partitions.
The MBR partition table restricts partition sizes to a maximum of 2.19 terabytes (2.19 × 1012 bytes) or almost exactly 2 TiB (2,199,023,255,040 bytes or 4,294,967,295 (232−1) sectors × 512 (29) bytes per sector).[2]. GPT supports partition sizes up to 9.3 zettabytes or 8 ZiB−512 bytes (9,444,732,965,739,290,426,880 bytes or 18,446,744,073,709,551,615 (264−1) sectors × 512 (29) bytes per sector).
Contents |
[edit] History
![[icon]](/wiki/File:Wiki_letter_w_cropped.svg){kind=small} |
This section requires expansion. |
The widespread MBR partitioning scheme, dating from the early 1980s, imposed limitations which affected the use of modern hardware. Intel therefore developed a new partition-table format in the late 1990s as part of what eventually became UEFI. The GPT as of 2010[update] forms a subset of the UEFI specification.[3]
[edit] Features
MBR-based partition table schemes insert the partitioning information in the master boot record (MBR) (which on a BIOS system is also the container for code that begins the process of booting the system). In a GPT, partition table information is stored in the GPT header, but to maintain compatibility, GPT retains the MBR entry as the first sector on the disk followed by a primary partition table header, the actual beginning of a GPT.
Like modern MBRs, GPTs use logical block addressing (LBA) in place of the historical cylinder-head-sector (CHS) addressing. Legacy MBR information is contained in LBA 0, the GPT header is in LBA 1, and the partition table itself follows. 64-bit Windows operating systems reserve 16,384 bytes (or 32 sectors at 512 bytes) for the GPT, leaving LBA 34 as the first usable sector on the disk.
Apple warns:[4] "Do not assume that the block size is always going to be 512 bytes." Modern storage devices such as solid-state drives may contain 1024-byte LBAs and some magneto-optical (MO) drives use 2048-byte sectors (but MO drives are typically not partitioned).
Hard disk manufacturers are transitioning to 4096-byte sectors. As of 2010, the first such drives continue to present 512-byte physical sectors to the OS, so degraded performance can result when the drive's (hidden) internal 4 KiB sector boundaries do not coincide with the 4 KiB logical blocks, clusters and virtual memory pages common in many operating systems and file systems. This is a particular problem on writes when the drive is forced to perform two read-modify-write operations to satisfy a single misaligned 4 KiB write operation.[5] Such a misalignment occurs by default if the first partition is placed immediately after the GUID partition table, as the next block is LBA 34. The next 4 KiB boundary begins with LBA 40.
Drives which boot Intel-based Macs must be formatted with a GUID Partition Table, rather than with the Apple Partition Map (APM).
GPT also provides redundancy, writing the GPT header and partition table both at the beginning and at the end of the disk.
Microsoft Windows implementations limit the number of possible partitions to 128.
[edit] Legacy MBR (LBA 0)
In the GPT specification, the location corresponding to the MBR in MBR-based disks is structured in a way that prevents MBR-based disk utilities from mis-recognizing, and possibly over-writing, GPT disks. This is referred to as a "protective MBR". In operating systems that support GPT-based boot, it is also used to store the first stage of the bootloader code. A single partition type of EEh, encompassing the entire GPT drive, is indicated and identifies it as GPT. Operating systems which cannot read GPT disks will generally recognize the disk as containing one partition of unknown type and no empty space, and will typically refuse to modify the disk unless the user explicitly requests and confirms the deletion of this partition. This minimizes accidental erasures. Furthermore, Apple's advice is that GPT aware tools should check the protective MBR and if the enclosed partition type is not of type EEh or if there are multiple partitions defined on the target device, the device should not be manipulated.[6]
If the disk exceeds 2 TiB—-the maximum partition size representable using the 32-bit LBAs of the legacy MBR (assuming a 512 byte block size)—-the size of this partition is marked as 2 TiB, ignoring the rest of disk.
Apple's Boot Camp Intel based Apple macs software creates a hybrid partition table to allow the booting of Windows (which at the time of Boot Camp's creation did not support GPT or EFI). In this system the protective partition is reduced in size to cover from sector 1 to the sector before the first regular partition included in the hybrid MBR. Additional MBR partitions are then defined to correspond to the next three GPT partitions.
[edit] Partition table header (LBA 1)
The partition table header defines the usable blocks on the disk. It also defines the number and size of the partition entries that make up the partition table. On 64-bit Windows Server 2003 machines, 128 partitions can be created. There are 128 partition entries reserved, each 128 bytes long. (The EFI specification requires that a minimum of 16,384 bytes be reserved for the partition table, so this gives space for 128 partition entries.)
The header contains the disk GUID (Globally Unique Identifier). It records its own size and location (always LBA 1) and the size and location of the secondary GPT header and table (always the last sectors on the disk). Importantly, it also contains a CRC32 checksum for itself and for the partition table, which may be verified by the firmware, bootloader and/or operating system on boot. Because of this, hex editors should not be used to modify the contents of the GPT. Such modification would render the checksum invalid. In this case, the primary GPT may be overwritten with the secondary one by disk recovery software. If both GPTs contain invalid checksums, the disk would be unusable.
| Offset | Length | Contents |
|---|---|---|
| 0 | 8 bytes | Signature ("EFI PART", 45h 46h 49h 20h 50h 41h 52h 54h) |
| 8 | 4 bytes | Revision (For GPT version 1.0 (through at least UEFI version 2.3.1), the value is 00h 00h 01h 00h) |
| 12 | 4 bytes | Header size in little endian (in bytes, usually 5Ch 00h 00h 00h meaning 92 bytes) |
| 16 | 4 bytes | CRC32 of header (0 to header size), with this field zeroed during calculation |
| 20 | 4 bytes | Reserved; must be zero |
| 24 | 8 bytes | Current LBA (location of this header copy) |
| 32 | 8 bytes | Backup LBA (location of the other header copy) |
| 40 | 8 bytes | First usable LBA for partitions (primary partition table last LBA + 1, usually 34) |
| 48 | 8 bytes | Last usable LBA (secondary partition table first LBA - 1, usually Disk-Size - 34) |
| 56 | 16 bytes | Disk GUID (also referred as UUID on UNIXes) |
| 72 | 8 bytes | Partition entries starting LBA (always 2 in primary copy) |
| 80 | 4 bytes | Number of partition entries |
| 84 | 4 bytes | Size of a partition entry (usually 128) |
| 88 | 4 bytes | CRC32 of partition array |
| 92 | * | Reserved; must be zeroes for the rest of the block (420 bytes for a 512-byte LBA) |
| LBA size | Total | |
The values for current and backup LBAs of the primary header should be the second sector of the disk (1) and the last sector of the disk, respectively. The secondary header at the end of the disk identifies its own table of partition entries, which is located directly before that header.
[edit] Partition entries (LBA 2–33)
The GPT uses simple and straightforward entries to describe partitions. The first 16 bytes designate the partition type GUID. For example, the GUID for an EFI System partition is {C12A7328-F81F-11D2-BA4B-00A0C93EC93B}. The second 16 bytes contain a GUID unique to the partition. Starting and ending 64-bit LBAs are also recorded here, and space is allocated for partition names and attributes. As is the nature and purpose of GUIDs, no central registry is needed to ensure the uniqueness of the GUID partition type designators.
| Offset | Length | Contents |
|---|---|---|
| 0 | 16 bytes | Partition type GUID |
| 16 | 16 bytes | Unique partition GUID |
| 32 | 8 bytes | First LBA (little-endian) |
| 40 | 8 bytes | Last LBA (inclusive, usually odd) |
| 48 | 8 bytes | Attribute flags (e.g. bit 60 denotes read-only) |
| 56 | 72 bytes | Partition name (36 UTF-16LE code units) |
| 128 | Total | |
Apple warns: "Do not hardwire the current size of the partition entry (128 bytes)."[7] Microsoft TechNet says that attributes are divided into two halves: the lower 4 bytes representing partition independent attributes, and the upper 4 bytes are partition type dependent. Microsoft uses the following bits in general:[8]
| Bit | Content |
|---|---|
| 0 | System partition (disk partitioning utilities must reserve the partition as is) |
| 2 | Legacy BIOS Bootable (equivalent to MBR 80h Active flag)[9] |
| 60 | Read-only |
| 62 | Hidden |
| 63 | Do not automount (i.e., do not assign drive letter) |
[edit] OS support of GPT
Hybrid MBRs are non-standard and can be interpreted in different ways by different OSes.[10] Unless otherwise noted, OSes provide precedence to the GPT data when a hybrid MBR configuration is encountered.
The term No native support on this arch and version. should be understood this way:
- Not supported as data disk[11], only known legacy partitions found in protective MBR are accessible via the OS. Detachable disks: only support for MBR partitioning; No access with end user applications. GPT contained raw data is accessible with third-party administrator tools for low level disk access. True file system level support in read or read-write form might be subject of software from a third-party vendor.
[edit] Unix-class operating systems
| OS | Version/ edition |
Platform | Boot from GPT on PC/BIOS | Boot from GPT on EFI | Note |
|---|---|---|---|---|---|
| FreeBSD | Since 7.0 | x86, x86-64 | Yes | Yes | In a hybrid configuration, both GPT and MBR partition identifiers may be used. |
| Linux | Most of the x86 Linux distributions |
x86, x86-64 | Yes | Yes | Some distribution tools, such as fdisk, don't work with GPT.
New tools such as gdisk,[13] GNU Parted,[14][15] Syslinux, grub .96+ patches and grub2 have been GPT-enabled. |
| Mac OS X | Since 10.4.0 (some features since 10.4.6)[16] | x86, x86-64 | Unofficial support (Hackintosh) | Yes | |
| MidnightBSD | Since 0.4-CURRENT | x86, x86-64 | Yes | No | In a hybrid configuration, both GPT and MBR partition identifiers may be used. |
| Solaris | Since Solaris 10 | x86, x86-64, SPARC | No (work in progress) | No (work in progress) |
[edit] Windows 32-bit versions
Microsoft does not support EFI on 32-bit platforms, and by extension, does not allow booting from GPT partitions.
| OS | Version/ edition |
Platform | Boot from GPT on PC/BIOS | Boot from GPT on EFI | Note |
|---|---|---|---|---|---|
| Windows XP | (2001-10-25) | x86 | No | No | No native support on this architecture and version. |
| Windows Server 2003 | (2003-04-24) | x86 | No | No | No native support on this architecture and version. |
| Windows Server 2003 | (2005-03-30) | x86 | No | No | Data Disk only [11] MBR takes precedence in hybrid configuration. |
| Windows Vista | (2005-07-22) | x86 | No | No | MBR takes precedence in hybrid configuration. |
| Windows Server 2008 | (2008-02-27) | x86 | No | No | MBR takes precedence in hybrid configuration. |
| Windows 7 | (2009-10-22) | x86 | No | No | MBR takes precedence in hybrid configuration.[10] |
[edit] Windows x64 64-bit versions
The following table lists only the 64-bit IA-32, also called x86-64, EM64T, or x64, Windows versions that support GPT. These are also since 2001 the "64-bit" editions for the IA-64 processor platform that come with EFI and support GPT boot since the first release, as GPT is the default partitioning type on that platform. The "x64" editions for the x86-64 processor platform initially only had limited GPT support until mainstream Windows variants reached parity with the IA-64 variants on the release of Windows Server 2008. Please note that below the so-called "Windows XP" version listed is in fact technically equivalent with the above-mentioned Windows Server 2003 version.
| OS | Version/ edition |
Platform | Boot from GPT on PC/BIOS | Boot from GPT on EFI | Note |
|---|---|---|---|---|---|
| Windows Server 2003 | (2005-04-25)[17] | x86-64 | No | No | Data disk only[11] MBR takes precedence in hybrid MBR configuration. |
| Windows XP | (2005-04-25) |
x86-64 | No | No | Data disk only[11] MBR takes precedence in hybrid configuration.
Detachable disks: only support for MBR partitioning. |
| Windows Vista | (2006-07-22) | x86-64 | No | No | MBR takes precedence in hybrid configuration. |
| Windows Server 2008 | (2008-02-27) | x86-64 | No | Yes | MBR takes precedence in hybrid configuration. |
| Windows 7 | (2009-10-22) | x86-64 | No | Yes | MBR takes precedence in hybrid configuration.[10] |
| Windows Server 2008 R2 | (2009-10-22) |
x86-64 | No | Yes | MBR takes precedence in hybrid configuration. |
[edit] Partition type GUIDs
| Assoc. OS | Partition type | Globally unique identifier (GUID)[1] |
|---|---|---|
| (None) | Unused entry | 00000000-0000-0000-0000-000000000000 |
| MBR partition scheme | 024DEE41-33E7-11D3-9D69-0008C781F39F | |
| EFI System partition | C12A7328-F81F-11D2-BA4B-00A0C93EC93B | |
| BIOS Boot partition[2] | 21686148-6449-6E6F-744E-656564454649 | |
| Windows | Microsoft Reserved Partition | E3C9E316-0B5C-4DB8-817D-F92DF00215AE |
| Basic data partition[3] | EBD0A0A2-B9E5-4433-87C0-68B6B72699C7 | |
| Logical Disk Manager metadata partition | 5808C8AA-7E8F-42E0-85D2-E1E90434CFB3 | |
| Logical Disk Manager data partition | AF9B60A0-1431-4F62-BC68-3311714A69AD | |
| Windows Recovery Environment | DE94BBA4-06D1-4D40-A16A-BFD50179D6AC | |
| IBM General Parallel File System (GPFS) partition | 37AFFC90-EF7D-4E96-91C3-2D7AE055B174 | |
| HP-UX | Data partition | 75894C1E-3AEB-11D3-B7C1-7B03A0000000 |
| Service Partition | E2A1E728-32E3-11D6-A682-7B03A0000000 | |
| Linux | Linux filesystem data[3] | EBD0A0A2-B9E5-4433-87C0-68B6B72699C7
0FC63DAF-8483-4772-8E79-3D69D8477DE4[citation needed] |
| RAID partition | A19D880F-05FC-4D3B-A006-743F0F84911E | |
| Swap partition | 0657FD6D-A4AB-43C4-84E5-0933C84B4F4F | |
| Logical Volume Manager (LVM) partition | E6D6D379-F507-44C2-A23C-238F2A3DF928 | |
| Reserved | 8DA63339-0007-60C0-C436-083AC8230908 | |
| FreeBSD | Boot partition | 83BD6B9D-7F41-11DC-BE0B-001560B84F0F |
| Data partition | 516E7CB4-6ECF-11D6-8FF8-00022D09712B | |
| Swap partition | 516E7CB5-6ECF-11D6-8FF8-00022D09712B | |
| Unix File System (UFS) partition | 516E7CB6-6ECF-11D6-8FF8-00022D09712B | |
| Vinum volume manager partition | 516E7CB8-6ECF-11D6-8FF8-00022D09712B | |
| ZFS partition | 516E7CBA-6ECF-11D6-8FF8-00022D09712B | |
| Mac OS X | Hierarchical File System Plus (HFS+) partition | 48465300-0000-11AA-AA11-00306543ECAC |
| Apple UFS | 55465300-0000-11AA-AA11-00306543ECAC | |
| ZFS[4] | 6A898CC3-1DD2-11B2-99A6-080020736631 | |
| Apple RAID partition | 52414944-0000-11AA-AA11-00306543ECAC | |
| Apple RAID partition, offline | 52414944-5F4F-11AA-AA11-00306543ECAC | |
| Apple Boot partition | 426F6F74-0000-11AA-AA11-00306543ECAC | |
| Apple Label | 4C616265-6C00-11AA-AA11-00306543ECAC | |
| Apple TV Recovery partition | 5265636F-7665-11AA-AA11-00306543ECAC | |
| Apple Core Storage (i.e. Lion FileVault) partition | 53746F72-6167-11AA-AA11-00306543ECAC | |
| Solaris | Boot partition | 6A82CB45-1DD2-11B2-99A6-080020736631 |
| Root partition | 6A85CF4D-1DD2-11B2-99A6-080020736631 | |
| Swap partition | 6A87C46F-1DD2-11B2-99A6-080020736631 | |
| Backup partition | 6A8B642B-1DD2-11B2-99A6-080020736631 | |
| /usr partition[4] | 6A898CC3-1DD2-11B2-99A6-080020736631 | |
| /var partition | 6A8EF2E9-1DD2-11B2-99A6-080020736631 | |
| /home partition | 6A90BA39-1DD2-11B2-99A6-080020736631 | |
| Alternate sector | 6A9283A5-1DD2-11B2-99A6-080020736631 | |
| Reserved partition | 6A945A3B-1DD2-11B2-99A6-080020736631 | |
| 6A9630D1-1DD2-11B2-99A6-080020736631 | ||
| 6A980767-1DD2-11B2-99A6-080020736631 | ||
| 6A96237F-1DD2-11B2-99A6-080020736631 | ||
| 6A8D2AC7-1DD2-11B2-99A6-080020736631 | ||
| NetBSD[5] | Swap partition | 49F48D32-B10E-11DC-B99B-0019D1879648 |
| FFS partition | 49F48D5A-B10E-11DC-B99B-0019D1879648 | |
| LFS partition | 49F48D82-B10E-11DC-B99B-0019D1879648 | |
| RAID partition | 49F48DAA-B10E-11DC-B99B-0019D1879648 | |
| Concatenated partition | 2DB519C4-B10F-11DC-B99B-0019D1879648 | |
| Encrypted partition | 2DB519EC-B10F-11DC-B99B-0019D1879648 | |
| ChromeOS[6] | ChromeOS kernel | FE3A2A5D-4F32-41A7-B725-ACCC3285A309 |
| ChromeOS rootfs | 3CB8E202-3B7E-47DD-8A3C-7FF2A13CFCEC | |
| ChromeOS future use | 2E0A753D-9E48-43B0-8337-B15192CB1B5E | |
| MidnightBSD[7] | Boot partition | 85D5E45E-237C-11E1-B4B3-E89A8F7FC3A7 |
| Data partition | 85D5E45A-237C-11E1-B4B3-E89A8F7FC3A7 | |
| Swap partition | 85D5E45B-237C-11E1-B4B3-E89A8F7FC3A7 | |
| Unix File System (UFS) partition | 0394Ef8B-237E-11E1-B4B3-E89A8F7FC3A7 | |
| Vinum volume manager partition | 85D5E45C-237C-11E1-B4B3-E89A8F7FC3A7 | |
| ZFS partition | 85D5E45D-237C-11E1-B4B3-E89A8F7FC3A7 |
- ^ The GUIDs in this table are written assuming a little-endian byte order. For example, the GUID for an EFI System partition is written as C12A7328-F81F-11D2-BA4B-00A0C93EC93B here, which corresponds to the 16 byte sequence 28h 73h 2Ah C1h 1Fh F8h D2h 11h BAh 4Bh 00h A0h C9h 3Eh C9h 3Bh — only the first three blocks are byte-swapped.
- ^ The formation of this GUID does not follow the GUID definition, it is formed by using the ASCII codes. Such formation of "GUID" value breaks down the guaranteed uniqueness of GUID.
- a b Previously Linux used the same GUID for the data partitions as Windows. Linux never had a separate unique partition type GUID defined for its data partitions. This created problems when dual-booting Linux and Windows in UEFI-GPT setup. This new GUID was defined jointly by GPT fdisk and GNU Parted developers. It is identified as type code 8300 in GPT fdisk.[citation needed]
- a b The GUID for
/usron Solaris is used as a generic GUID for ZFS by Mac OS X. - ^ Definitions are in src/sys/sys/disklabel_gpt.h. NetBSD had used the FreeBSD GUIDs before unique NetBSD-specific GUIDs were created.
- ^ Definitions are in http://www.chromium.org/chromium-os/chromiumos-design-docs/disk-format.
- ^ Definitions are in [2]. MidnightBSD had used the FreeBSD GUIDs before unique GUIDs were created.
[edit] See also
- Master boot record
- Globally Unique Identifier (GUID)
- Extensible Firmware Interface (EFI)
- Disk partitioning
[edit] References
- ^ Bill Boswell (2002-07-01). "FAQ: Drive Partition Limits". Redmondmag.com. http://redmondmag.com/Articles/2002/07/01/The-64Bit-Question.aspx. Retrieved 2010-06-09. "GPT disks also support very large partitions thanks to a 64-bit Logical Block Address scheme. A logical block corresponds to one sector, or 512 bytes, yielding a maximum theoretical capacity of eight zettabytes."
- ^ "FAQ: Drive Partition Limits" (pdf). UEFI Forum. http://www.uefi.org/learning_center/UEFI_MBR_Limits_v2.pdf. Retrieved 2010-06-09.
- ^ Nikkel, Bruce J. (September 2009). "Forensic analysis of GPT disks and GUID partition tables". Digital Investigation 6 (1-2): 39–47. "The current popular BIOS and MBR partitioning scheme was originally developed in the early 1980s for the IBM Personal Computer using IBM PC-DOS or MS-DOS. The Basic Input/Output System (BIOS) provides an interface to the hardware and initiates the boot process (IBM, 1983). The MBR, located in sector zero, contains the initial boot code and a four entry partition table (Microsoft, 1983). Intended to solve booting and partitioning limitations with newer hardware, a replacement for both the BIOS and the MBR partition table was developed by Intel in the late 1990s (Intel, 2000). This is now called the Unified EFI (UEFI, 2008 UEFI Forum. Unified extensible firmware interface specification version 2.2 2008.UEFI, 2008) specification, and managed by the UEFI Forum (UEFI, 2009). A subset of this specification includes GUID (globally unique identification) Partition Tables, or GPT, intended to replace the DOS/MBR partition tables."
- ^ "Technical Note TN2166: Secrets of the GPT". Apple. http://developer.apple.com/technotes/tn2006/tn2166.html.
- ^ "Western Digital’s Advanced Format: The 4K Sector Transition Begins". Anandtech. http://anandtech.com/storage/showdoc.aspx?i=3691.
- ^ http://developer.apple.com/library/mac/technotes/tn2166/
- ^ http://developer.apple.com/mac/library/technotes/tn2006/tn2166.html#SECGPTOVERVIEW
- ^ http://technet.microsoft.com/en-us/library/cc739412%28WS.10%29.aspx#w2k3tr_basic_how_fgkm
- ^ "e09127r3 EDD-4 Hybrid MBR boot code annex". http://t13.org./documents/UploadedDocuments/docs2010/e09127r3-EDD-4_Hybrid_MBR_boot_code_annex.pdf.
- ^ a b c Smith, Rod. "Hybrid MBRs: The Good, the Bad, and the So Ugly You'll Tear Your Eyes Out". http://www.rodsbooks.com/gdisk/hybrid.html.
- ^ a b c d "Windows and GPT FAQ". Microsoft. http://www.microsoft.com/whdc/device/storage/GPT_FAQ.mspx#ELD.
- ^ "Ubuntu on MacBook". Ubuntu Community Documentation. https://help.ubuntu.com/community/MacBook.
- ^ Smith, Rod. "GPT fdisk for Linux". http://www.rodsbooks.com/gdisk.
- ^ "GNU Parted FAQ". https://www.gnu.org/software/parted/faq.shtml#features.
- ^ "mklabel - GNU Parted Manual". https://www.gnu.org/software/parted/manual/html_node/mklabel.html.
- ^ "Myths and Facts About Intel Macs". rEFIt. http://refit.sourceforge.net/myths/.
- ^ Microsoft Raises the Speed Limit with the Availability of 64-Bit Editions of Windows Server 2003 and Windows XP Professional [1]
[edit] External links
- Microsoft TechNet: Disk Sectors on GPT Disks (Archived page)
- Microsoft TechNet: Troubleshooting Disks and File Systems
- Microsoft TechNet: Using GPT Drives
- Microsoft: FAQs on Using GPT disks in Windows
- Apple Developer Connection: Secrets of the GPT
- Make the most of large drives with GPT and Linux
- GPT fdisk: Information on Hybrid GPT-MBR, Converting MBR and BSD disklabels to GPT and Booting from GPT disks
- Convert Windows Vista SP1+ or 7 x86_64 boot from BIOS-MBR mode to UEFI-GPT mode without Reinstall
- Rod Smith - A BIOS to UEFI Transformation
- Support for GPT (Partition scheme) and HDD greater than 2.19 TB in Microsoft Windows XP
- Setting up a RAID volume in Linux with >2TB disks
