HP D212 Manuale utente

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 512-

byte 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.

Figure 1. GPT partition table scheme

Figure 2 on the following page shows a comparison of the MBR and GPT layouts.

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

Protects MBR-based

disk utilities

from overwriting

GPT drives

Defines usable

blocks on the disk

List of drive

partition entries

(up to 128)

Drive partitions

(up to 128)

Secondary/backup

GPT header and

partition entries

LBA = 512 bytes

NOTE: LBA -34 refers to the 34th addressable block

address counted backwards from the last LBA on

the drive.

Primary GPT

Secondary GPT

LBA 0

LBA 1

LBA 2

LBA 3

LBA -34

LBA 34

LBA -33

LBA -2

LBA -1

Figure 2. Comparison of MBR and GPT disk layouts

Bootable vs Data Drives and UEFI BIOS

Since the partitions on a data-storage drive are accessed by the OS and not the

BIOS, GPT drives can be used on non-UEFI systems as data-storage drives. These

drives cannot, however, be used as boot drives in such systems. The legacy BIOS in

a non-UEFI system cannot find the boot information on a GPT-formatted drive and

therefore cannot complete the boot process into the OS. In using a GPT drive as a

data-storage drive, the drive must be formatted using tools that can create GPT

partitions.

In order to boot from a GPT-formatted drive, the system must have a UEFI BIOS.

UEFI understands how to find the boot information on a GPT drive. In addition, the

system must also have a “UEFI aware” OS such as Microsoft Windows 7 ™64-bit.

For selected desktops, HP implements the Type 0 BIOS Information data structure

(as described in version 2.7 of the SMBIOS specification) to indicate UEFI BIOS

support. The BIOS Characteristics Extension Bytes field within this structure has

defined a bit that, if set, identifies the system as UEFI compliant. Table 1 describes

BIOS Characteristic Extension Byte 2, with Bit 3 identified as the bit for UEFI

specification support.

Master Boot Code

1st Partition Table Entry

2nd Partition Table Entry

3rd Partition Table Entry

4th Partition Table Entry

0x55 AA

Primary GUID Partition

Table Header

GUID Partition Entry 1

GUID Partition Entry 2

GUID Partition Entry n

GUID Partition Entry 128

Primary Partition (C:)

Primary Partition (E:)

Primary Partition n

GUID Partition Entry 1

GUID Partition Entry 2

GUID Partition Entry n

GUID Partition Entry 128

Backup GUID

Partition Table Header

Master Boot Record

Master Boot Code

1st Partition Table Entry

2nd Partition Table Entry

3rd Partition Table Entry

4th Partition Table Entry

0x55 AA

Primary Partition (C:)

Primary Partition (E:)

Primary Partition (F:)

Logical Drive (G:)

Logical Drive (H:)

Logical Drive n

Protective MBR

Partition Table

Extended

Partition

Primary GUID

Partition Entry

Array

Backup GUID

Partition Entry

Array

Basic GPT Disk LayoutBasic MBR Disk Layout

The BIOS Characteristic Extension Byte #2 is described in Table 1. Bit 3 of Byte 2

specifies UEFI specification support.

Table 1. BIOS Characteristics Extension Byte 2

Bit Position Description

0 When set, BIOS boot specification is supported

1 When set, function key-initiated Network Service boot is supported. When

function key-initiated Network Service boot is not supported, a network

option ROM may choose to offer this functionality on its own, thus offering

this capability to legacy systems. When the function is supported, the

network adapter option ROM shall not offer this capability.

2 When set, enables Targeted Content Distribution. The manufacturer has

ensured that the SMBIOS data is useful in identifying the computer for

targeted delivery of model-specific software and firmware content through

third-party content distribution services.

3 When set, UEFI specification is supported.

4 When set, the SMBIOS table describes a virtual machine. When cleared, no

inference can be made about the virtuality of the system.

5:7 Reserved for future assignment by this specification.

The user can determine if the system has a UEFI BIOS by looking for the splash

screen during boot referencing a top-level Startup menu (Figure 3).

Figure 3. Splash screen of HP UEFI BIOS system

The startup menu (Figure 4) can be accessed by pressing the Esc key during the

boot sequence.

Figure 4. Startup Menu

OS Support

32-bit Microsoft Windows™ will not provide boot support for GPT. For HP desktops,

bootable GPT drives and data storage drives will be initially supported under the

Microsoft Windows 7™ SP1 64-bit OS. Bootable GPT drives can also be supported

under the Microsoft Vista™ SP1 64-bit OS. Both of these operating systems require

a UEFI BIOS to support a bootable GPT drive. Data storage-only GPT drives will be

supported under 32-bit and 64-bit Microsoft Windows 7™ SP1 and can be

supported under 32-bit and 64-bit Microsoft Windows Vista™. At this time, there

are no immediate plans for HP Desktops to directly support GPT drives under

Linux™ (HP Workstations have separate OS plans). Table 2 summarizes current OS

support for HP Desktops.

Table 2. Summary of OS support for GPT drives by HP Desktops

Operating system GPT Boot Support? GPT Data Storage Support?

Windows 7™ SP1 64-bit Yes (requires UEFI BIOS) Yes

Windows 7™ SP1 32-bit No Yes

Windows Vista™ SP1 64-bit Yes (requires UEFI BIOS) Yes

Windows Vista™ SP1 32-bit No Yes

Linux™ No direct support from HP No direct support from HP

Storage Driver Support

Since GPT drives have capacities beyond 2.2TB, support is required through

updated storage drivers. Storage driver support can be provided with certain newer

operating systems’ AHCI (Advanced Host Controller Interface) drivers for larger

drive capacities. For improved performance, HP Desktops support GPT hard drives

with storage device drivers from AMD™ and Intel™. These drivers will support both

single drive implementation in AHCI and RAID 0 (striped) and RAID 1 (mirrored)

formats. Expected timeframe for AHCI support is 1H 2011. Support for GPT drives

in bootable RAID configurations is expected in 2H 2011.

Imaging Tool Support for GPT

In generating SW boot images for GPT drives, the user must utilize “GPT aware”

software tools. Not all tools will automatically support partitioning and formatting

for GPT. For example, only newer versions of Symantec GHOST™ are able to

handle GPT partition tables. The user will need to check the particular tool

documentation to ensure the software provides support for GPT.

A certain percentage of users may choose to re-install the 64-bit OS with a

Microsoft DVD. On machines with UEFI BIOS, the DVD setup utility may try to install

the image in GPT format by default. If this is not what the user wants, then the DVD

setup utility will need to be manually selected to install in MBR format instead. To

install in MBR mode, the user will need to access the boot menu using F9, then

select “Legacy Boot Sources/ATAPI DVD Drive/SATA0” as shown in Figure 5.

Figure 5. BIOs menu of HP UEFI BIOS system

Drive Encryption

For Opal-based SED (Self Encrypting Drive) hard drives, UEFI protocol definitions

and usage for SED are part of the UEFI v2.3.1 Spec published in Feb 2011. To

ensure proper operation of Opal-based SED drives with GPT formatting, users will

need to useUEFI BIOS and SED drives that conform to this version or to later

versions of the UEFI specification. For all types of SED drives, the only other major

requirement is that the “Shadow MBR” used in SED drives must not exceed 128MB.

Since the GPT partition table is normally 15KB, this should not present a problem.

Other drive encryption tools, such as McAfee Endpoint™ software encryption tools,

may or may not have specific GPT support at this point. Consult the particular

software vendor for details regarding planned GPT support.

Summary

GPT-based hard disk drives offer a more consistent, scalable approach to

formatting and partitioning hard drives. GPT allows more efficient use of the larger

hard drives now becoming available, and also allows such hard drives to be a

bootable part of a UEFI-based system. With GPT, partition and formatting support

of ever-increasing drive sizes is assured for many more years.

For more information

Additional information on topics mentioned in this document is available at the

following websites:

For the UEFI specification, go to the UEFI SIG website at:

http://www.uefi.org/home/

is subject to change without notice. The only warranties for HP products and services

are set forth in the express warranty statements accompanying such products and

services. Nothing herein should be construed as constituting an additional warranty. HP

shall not be liable for technical or editorial errors or omissions contained herein. Intel is

a trademark of Intel Corporation in the U.S. and other countries. Microsoft and

Windows are U.S. registered trademarks of Microsoft Corporation. Microsoft and

Windows are U.S. registered trademarks of Microsoft Corporation.

660177--001, April 2011

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