What's a GPT?
Last revision: 6/26/2010, GPT fdisk version 0.7.2
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Note: This page is part of the documentation for my GPT fdisk program.
In order to understand what GPT is, you must first understand the
current standard for disk partitioning and its limits. With that knowledge
in hand, you can see how GPT can fix MBR's deficiencies.
MBR, Its Annoyances, and Its Limits
Since the earliest hard disks for x86 computers, these disks have been
divided into one or more partitions using a partitioning scheme that has,
through the ages, gone by several names, such as MS-DOS
disklabels, BIOS partitions, and MBR partitions. By
whatever name, the MBR partitioning scheme has several characteristics that
have, in one way or another, been limitations or annoyances:
- The MBR system originally provided for only four partitions. As a
workaround, one of these original (or primary) partitions can
be set aside (and is then known as an extended partition) to
hold an arbitrary number of logical partitions. This
configuration is awkward and can lead to problems, since some OSes can
only boot from primary partitions and since it's not easy to convert a
partition from one type to another. (The FixParts program, which
is part of the GPT fdisk family, can convert between primary and
logical types, within certain limits.)
- MBR partitions include one-byte partition type codes to help OSes
identify the partitions' purpose. The single byte has proven somewhat
limiting, and there are occasional collisions—a single code with an
ambiguous meaning, such as 0x82, which can refer to either Linux swap
space or a Solaris disklabel.
- The MBR scheme originally employed cylinder/head/sector (CHS) addresses
to identify partitions' locations. This feature can lead to problems
because different BIOSes or OSes can interpret the CHS geometry in
different ways. The CHS scheme also tops out at a maximum disk size of
about 8 GiB. In part to work around this limitation, MBR has been
extended to use 32-bit logical block addressing (LBA), but using two
systems itself introduces the opportunity for problems, such as
mismatched CHS and LBA definitions for a single partition. In practice,
modern OSes seem to mostly ignore the CHS values, and they
must ignore the CHS values on partitions that begin or end
beyond the 8 GiB mark.
- In conjunction with the near-universal sector size of 512 bytes, the
32-bit LBA pointers used by MBR partitions impose a 2 TiB limit on disk
and partition size. This limit, unlike the earlier 8 GiB limit of CHS,
is not easily overcome by a simple extension to MBR; there simply isn't
space left in the size allocated to MBR data structures. With 3 TB
drives available now and even larger devices configurable using RAID
technologies, this problem is a very serious one and will only become
more serious with time.
- MBR partitions are susceptible to damage. The primary partition table
is stored entirely within the first sector of the disk, so if it's
destroyed or damaged, it will be hard to recover the disk's partitions.
Logical partitions are stored in a linked list data structure that's
scattered over the extended partition, so if a single link is broken,
access to the remaining logical partitions will be lost.
There are techniques you can use to extend
the life of MBR; however, these methods are stop-gaps at best. Sooner
or later, you'll find MBR to be inadequate as you move to larger and larger
GPT to the Rescue
The heir apparent to MBR is GPT. This new partitioning scheme fixes many
of MBR's problems:
- In its most common configuration, GPT supports up to 128 partitions,
so there's no need for extended or logical partitions. In case needs
change, GPT data structures permit larger partition table sizes,
although in practice few tools seem to allow users to adjust this
parameter. (GPT fdisk does, though.)
- The GPT partition type code is a 128-bit (16-byte) GUID. In theory,
this gives plenty of room for lots of unique partition types.
Unfortunately, in practice there are already collisions—most
importantly for Linux users, Linux and Windows have used the same type
code for their filesystem partitions, although a new Linux-specific
code is now available. To supplement the type code, GPT supports a
plain-text name for each partition.
- GPT knows nothing about CHS geometries and it uses 64-bit sector
pointers, so 2 TiB drives are puny compared to the total supported disk
size of GPT.
- GPT adds CRC32 checksums to its data structures and stores those
structures twice on the disk—once at the start of the disk and again
at the end. These measures help protect the system against accidental
damage caused by carelessness or disk errors.
Unfortunately, GPT is not without its problems. These mainly relate to
compatibility. Older OSes have no or limited GPT support. For instance,
Windows only supports GPT at all on Windows Server 2003, the 64-bit (but
not the 32-bit) version of Windows XP, Windows Vista, and later. Through
Windows 7, booting from a GPT disk is impossible unless the system uses the
Unified Extensible Firmware Interface (UEFI) rather than a legacy BIOS.
(Most computers through 2010 still used a legacy BIOS, although by mid-2011
UEFI-based PCs started becoming common in stores.) Linux has long supported
GPT, but boot support depends on the boot loader. GRUB through version 0.97
doesn't officially support booting from GPT, but patched versions of GRUB
0.97 with GPT support are common, and GRUB2 officially supports booting
from GPT. Intel-based Macs use GPT by default. (The "Booting from GPT" section of this Web page
describes GPT boot issues in more detail.)
To protect GPT disks against errant older disk tools, GPT keeps an MBR
partition table on the first sector of the disk. This MBR contains a single
disk-spanning partition of type 0xEE, which makes older tools think the
disk is in use by an unknown OS. Some tools take advantage of this feature
to create a hybrid MBR configuration, in which
some partitions are accessible via both GPT and MBR definitions. Although
this is non-standard, awkward, delicate, and downright dangerous, it can
help make the transition from MBR to GPT easier by providing a workaround
for OSes that don't understand GPT.
In addition to the protective MBR, GPT features two main types of data
structure, each of which is stored on the disk twice:
- The GPT headers contain disk meta-data—information on the
locations of the partition tables, the disk GUID, and so on. No actual
partitions are defined in the headers, though. Each disk has two
headers, a main header and a backup header. If one is damaged, you can
use the other to recover the damaged one.
- Just as there are two headers, there are two partition tables.
These should be byte-for-byte identical to one another, unlike the
headers, which vary because they need to point to each other and to
their own partition tables.
Because of the hard 2 TiB limit of MBR partitions, chances are you'll be
forced to switch to GPT for at least some disks in the not-too-distant
future. MBR is likely to remain useful on smaller devices, such as USB
flash drives, for years to come. In 2011, most existing x86 and x86-64 PCs
still use MBR-partitioned disks, although as noted earlier, UEFI-based PCs
are now common in stores. Mac hardware is an exception to this rule; Apple
has embraced GPT, as well as the EFI, of which the GPT definition is
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