GUID Partition Table
GUID Partition Table (GPT ) to German GUID partition table ( from English- Globally Unique Identifier ) is a standard for the format of partition tables on disk or other media. The specification is part of the UEFI standard, which replaced in personal computers from mainframes around since 2000 increasingly the BIOS. GPT is the successor to the Master Boot Record ( MBR) partition tables. GUID partition tables can be used only under limitations with previous BIOS versions.
Construction
According to the GPT disk one of the following areas of:
- MBR whose special configuration allows the use of the plate under MBR operating systems and protects against changes due to MBR partitioning tools
- Primary GPT header and partition consisting of entries
- Partitions
- Secondary GPT header and partition consisting of entries
The secondary GPT at the end of the disk is partly a copy of the primary GPT at the beginning of the disk: the contents of the fields for the positions of their own and alternate GPT header are reversed and the address of the partition table refers to the copy of the partition table at the end of the plate before the alternate header. Thus, both GPT header also have a different CRC32 checksum. Due to the redundancy contained the partition table can be restored in case of failure. As in the GPT, a checksum is entered, it can be determined whether and which of the two GPT is wrong.
MBR partition table
In the first sector / block of the disk (LBA 0), the MBR is a classic MBR partition table and an entry that identifies the rest of the media as busy. For a partitioning tool can only read MBR but no GPT partition tables, therefore, appears all the space on the disk to be occupied. The MBR of a GPT disk is therefore a protection for the contents of the disk, if this partitioning with tools that do not yet know the GPT to be accessed (hence English protective MBR). A partition identifier of the value EE16 is a Protective MBR ( with subsequent GUID partition table) or EF16 used in an EFI system partition.
Header GUID partition table
In the second sector / block (LBA 1) is the primary GPT header and the last sector / block (LBA -1) is the secondary backup GPT. The header itself describes, inter alia, the usable blocks of the disk, the position of your header and the secondary backup header and the number and size of the partition entries. The EFI specifications stipulate a minimum of 16 384 bytes for the partition table so that there is space for 128 entries. Furthermore, the header itself and the partition entries are protected by CRC32 checksums that need to be checked by the firmware, the bootloader or operating system. Therefore, if the header, for example, changed with a hex editor, this makes the checksum invalid and the system becomes unstable or even unusable.
In the header of the GPT following information is stored:
Partition entry
The partition entries are to LBA 33 (secondary header: LBA 33 to LBA -2) in LBA 2 saved. Each partition entry is 128 bytes. Thus, four partition entries can be stored per logical block. ( This prescribed in the specification minimum of 32 sectors for partition entries can be increased as and when required so that the maximum possible number of partitions is in principle limited only by the available disk space. )
In the Partition entry itself following data are stored:
The individual entries are relatively simple. The first 16 bytes to describe the type of partition. The partition type GUID for an EFI system partition is eg { C12A7328 - F81F - 11D2 - BA4B - 00A0C93EC93B }. This is followed by 16 bytes with partition GUID. Then the number of the first and the last LBA sector of the partition 8 bytes with attributes and the name of the partition follow (36 Unicode characters long).
Microsoft defines the following attribute fields:
Partition type GUIDs
Limitations
The GPT uses Logical Block Addressing ( LBA) with 64-bit comprehensive entries, so ( in 512-byte sector size ) HDD addressed to a total size of 8 Zebibyte and these can be divided into any number of partitions.
Support in operating systems
For operating systems that build on the IA64 architecture (mainly mainframe ), support for GPT is imperative because these computers instead of using the BIOS UEFI booting. This includes the corresponding IA64 versions of FreeBSD, HP- UX, Linux, NetBSD, OpenVMS and Microsoft Windows. Mac OS X also supports it since conversion of the Apple Macintosh to Intel processors in early 2006, and has been available since the beginning of 2005, adjusted for the IA64 version of Solaris.
The versions of Microsoft Windows XP that have been developed for the 32 -bit Intel i386 architecture, on the other hand can be installed and operated on a GPT disk with some restrictions. Among other things, they only read the MBR, but not the GPT to obtain data partitioning. But there are in the MBR of GPT disks up to three primary partitions available, since the MBR does not allow more than four entries, of which the first partition is already reserved for the EFI firmware. In addition, extended partitions, which can contain multiple logical partitions as a container, no longer supported by GPT.
Until the appearance of Windows Vista in early 2007, the 64 -bit versions of Windows aimed primarily at admin and professional users (eg CAD or Graphics / Design). This includes the already released in 2005 Windows XP Professional x64 Edition. In their appearance the GPT support comprehensive was present by appropriate 64 -bit drivers only at selected hardware. With the publication of the availability of Vista 64- bit drivers was generally better, so they usually support also the x64 version of XP.
The trade magazine c't was able to find large, partitioned with GPT disk in the initial phase of implementation during the test of a 4 TB that numerous hard disk driver from well-known manufacturers when writing to areas beyond 2 TB took off the data held in the desired position at the beginning of the hard disk so that there was massive loss of data or even the loss of formatting the hard drive. The problem occurred apparently are particular to the 32- bit versions of Windows XP, although some already led support for GPT with it, but because of their 32 -bit driver designs in the earlier usual physical sector size of 512 bytes, the 2 - TB- limit could not exceed. Since the use of such partitions was still mainly to be found in servers, these problems were most likely when the hard drives of such a system have been replaced with other computers.
A 64 -bit Linux can - without UEFI BIOS support or separate - with GRUB2 boot from a GPT partition. So it is necessary not to UEFI - the coupling of GPT to (U) EFI is therefore not mandatory.
Swell
Acorn
Apple Macintosh ( m68k ) | Apple Macintosh ( PowerPC)
Alpha ( OSF disklabel ) | BSD | SGI (SGI disk label ) | Sun Microsystems (Sun disk label )
Apple Macintosh ( IA- 32) | disklabel (IA- 32) | IA-32 PC (UEFI ) | Itanium (IA -64)
Disklabel (IA- 32) | IBM - PC compatible (BIOS)
Amiga
Disk label
- Operating system component