Answer Posted / gvk@sysnetts.com
n computer hardware, GUID Partition Table (GPT) is a
standard for the layout of the partition table on a
physical hard disk. It is a part of the Extensible Firmware
Interface (EFI) standard proposed by Intel as a replacement
for the obsolescent PC BIOS, one of the few remaining
relics of the original IBM PC. EFI uses GPT where BIOS uses
a Master Boot Record (MBR).
Diagram illustrating the layout of the GUID Partition Table
scheme. Each logical block (LBA) is 512 bytes in size. LBA
addresses that are negative indicate position from the end
of the volume, with −1 being the last addressable block.
Diagram illustrating the layout of the GUID Partition Table
scheme. Each logical block (LBA) is 512 bytes in size. LBA
addresses that are negative indicate position from the end
of the volume, with −1 being the last addressable block.
Contents
[hide]
* 1 Features
* 2 Legacy MBR (LBA 0)
* 3 Partition table header (LBA 1)
* 4 Partition entries (LBA 2–33)
* 5 Partition type GUIDs
* 6 See also
* 7 External links
[edit] Features
Current PC BIOS schemes use a Master Boot Record (MBR) to
begin the process of initializing the disk. The MBR begins
with an entry called the Master Boot Code, which contains
an executable binary for the purpose of identifying and
booting the active partition. PC BIOS's replacement, EFI,
itself contains this capability but to maintain backwards
compatibility, GPT retains the MBR entry as the first
sector on the disk followed by a Primary Partition Table
Header, the actual beginning of GPT.
GPT uses modern logical block addressing (LBA) in place of
the cylinder-head-sector (CHS) addressing used with MBR.
Legacy MBR information is contained in LBA 0, the GPT
header is in LBA 1, and the partition table itself follows.
In 64-bit Windows operating systems, 16,384 bytes, or 32
sectors, are reserved for the GPT, leaving LBA 34 as the
first usable sector on the disk.
GPT also provides for redundancy. The GPT header and
partition table are written at both the beginning and end
of the disk.
[edit] Legacy MBR (LBA 0)
The primary purpose of the MBR at the beginning of the disk
is to prevent MBR-based disk utilities from mis-
recognizing, and possibly over-writing, GPT disks. A single
partition, encompassing the entire GPT drive, is indicated.
The System ID for the partition is set to 0xEE, indicating
that it uses GPT. Because of this, EFI ignores the MBR.
Some 32-bit OSes which cannot read GPT disks nevertheless
recognize this ID and present the disk as an inaccessible
GPT disk. Older OSes will generally recognize the disk as
containing one partition of unknown type and no empty
space, and then they'll typically also refuse to modify the
disk unless the user explicitly requests and confirms the
deletion of this partition. This way, accidental erasures
are prevented.
[edit] Partition table header (LBA 1)
The partition table header defines the blocks on the disk
that can be utilized by the user (the usable blocks). It
also defines the number and size of the partition entries
that make up the partition table. On 64-bit Windows Server
2003 machines, there are 128 partition entries reserved,
each 128 bytes long. Thus, 128 partitions can be created.
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 is verified by EFI
processes on boot. Because EFI uses and verifies this
checksum, hex editors should not be used to modify the
contents of the GPT. Such modification would render the
checksum invalid. In this case, EFI would overwrite the
primary GPT with the secondary one, or, if both GPTs
contained invalid checksums, would be unable to access the
disk.
[edit] Partition entries (LBA 2–33)
Partition entries are simple and straightforward. 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. Unfortunately, there is no
central registry for GUID partition type designators,
either de jure or de facto.
GUID partition entry format 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
[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 21686148-6449-6E6F-744E-656564454649
Windows Microsoft Reserved Partition E3C9E316-
0B5C-4DB8-817D-F92DF00215AE
Basic Data Partition[2] 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
HP-UX Data partition 75894C1E-3AEB-11D3-B7C1-7B03A0000000
Service Partition E2A1E728-32E3-11D6-A682-7B03A0000000
Linux Data partition[2] EBD0A0A2-B9E5-4433-87C0-
68B6B72699C7
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 (HFS+) partition
48465300-0000-11AA-AA11-00306543ECAC
Apple UFS 55465300-0000-11AA-AA11-00306543ECAC
ZFS[3] 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
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[3] 6A898CC3-1DD2-11B2-99A6-080020736631
/var partition 6A8EF2E9-1DD2-11B2-99A6-080020736631
/home partition 6A90BA39-1DD2-11B2-99A6-080020736631
EFI_ALTSCTR 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[4] 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
1. ^ 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 28 73 2A C1 1F F8 D2 11 BA 4B 00 A0 C9 3E C9 3B —
only the first three blocks are byte-swapped.
2. a b Linux and Windows use the same GUID for their
respective data partitions.
3. a b The GUID for /usr on Solaris is used as a
generic GUID for ZFS by Mac OS X.
4. ^ Definitions are in src/sys/sys/disklabel_gpt.h.
NetBSD had used the FreeBSD GUIDs before unique NetBSD-
specific GUIDs were created.
[edit] See also
* Master boot record
* GUID
* EFI
* Disk partitioning
[edit] External links
* Microsoft TechNet: Disk Sectors on GPT Disks
* Microsoft TechNet: Using GPT Drives on x86-64 Systems
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