HP 100B, 1105, 18, 200 G1, 202 G1 White Paper
...
GPT hard Disk Drives
For HP Desktops
April 2011
Table of Contents:
Abstract .............................................................................................................. 1
Why GPT? ........................................................................................................... 1
GPT vs MBR ........................................................................................................ 2
Bootable vs Data Drives and UEFI BIOS ................................................................ 4
OS Support ......................................................................................................... 6
Storage Driver Support ........................................................................................ 6
Imaging Tool Support for GPT .............................................................................. 7
Drive Encryption .................................................................................................. 7
Summary ............................................................................................................ 8
For more information .......................................................................................... 8
Abstract
In order to be used in a computer system, a hard disk drive (HDD) must first be
partitioned. Partitioning is done at the drive level and for many decades used the
Master Boot Record (MBR) partition method. A new method called GUID Partition
Table (GPT)) is now used for large-capacity drives. GPT allows for volume sizes
much greater than 2.19TB, and therefore must be used on drives larger than the
MBR limit of 2.19TB. This white paper describes the rationale for GPT and the types
of OS, BIOS, driver, and SW tool support needed to successfully implement GPT
drives in desktop systems.
Why GPT?
HDD areal density (bits/sq in) and tracks per inch (TPI) continue to steadily
increase. For 2010, many typical desktop 3.5” hard drives have areal densities
and TPI that permit capacities up to 500GB per platter. In 2011, it is expected that
improvements in areal density and TPI will allow drive manufacturers to begin
shipping drives that have capacities of1TB/platter. At this density, drives of 2TB
and 3TB capacities become more easily producible and hence cheaper and more
widely available.
For decades, MBR partitioning was sufficient to support the amount of hard drive
addressing spaces known as Logical Block Addresses (LBAs) that existed on hard
drives. MBR partitioning supported 32-bit addressing and, along with 48-bit LBA
extensions, was able to support drives up to a limit of 2.19TB using standard 512byte sectors (or 512-byte sector emulation). However, now that hard drive sizes are
beginning to exceed the 2.19TB limit, a newer approach is needed. GPT
partitioning, which is part of the overall Unified Extensible Firmware Interface (UEFI)
specification, allows for much larger hard drives than with MBR and will also allow
such a drive to be bootable in a system with UEFI BIOS and UEFI-aware OS.
GPT vs MBR
There are a number of differences between MBR and GPT. Some of the key
differences include the following:
• MBR only supports 4 primary partition table entries while GPT supports 128
primary partitions with current Microsoft Windows™ operating systems.
• MBR uses 32-bit values to indicate the starting offset and length of a partition.
Assuming 512 byte sectors or 512 byte sector emulation, this means an MBR
disk supports 2^32 bit LBAs multiplied by 512 bytes/sectors = 2.19 x10^12
bytes = 2.19TB maximum disk size.
• Since GPT uses 64-bit values to indicate the starting offset and length of a
partition, a GPT disk can theoretically support up to 2^64 LBAs. Assuming 512
byte sectors or 512 byte sector emulation, this means the maximum capacity of
a GPT disk = 9.4 x 10^21 bytes = 9.4 zettabytes (ZB). Microsoft Windows™
file systems design currently limits this to 256TB per drive
• GPT provides redundancy by writing the GPT header and partition table at the
beginning of the disk and also at the end of the disk. If the original GPT
header and partition table gets deleted or corrupted, the backup header and
partition table can be used to help restore the drive. The contents of the GPT
header and partition table are also verified via a CRC32 checksum for the
header and one for the partition table.
Figure 1 on the following page shows a diagram of the GPT Partition Table and
drive layout.
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blocks on the disk
LBA = 512 bytes
the drive.
Primary GPT
Secondary GPT
LBA 0
LBA 1
LBA 2
LBA 3
LBA -34
LBA 34
LBA -33
LBA -2
LBA -1
Protects MBR-based
disk utilities
from overwriting
GPT drives
Defines usable
List of drive
partition entries
(up to 128)
Drive partitions
(up to 128)
Secondary/backup
GPT header and
partition entries
Protective MBR
Primary GPT Header
Entries 1 thru 4
Entries 5 – 128
Partition 1
Partition 2
Remaining Partitions
Entries 1 thru 4
Entries 5 – 128
Secondary GPT Header
NOTE: LBA -34 refers to the 34th addressable block
address counted backwards from the last LBA on
Figure 1. GPT partition table scheme
Figure 2 on the following page shows a comparison of the MBR and GPT layouts.
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