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MBR and GPT Interview Questions

> What is a GPT disk?
The GUID Partition Table (GPT) was introduced as part of the Unified Extensible Firmware Interface (UEFI) initiative. GPT provides a more flexible mechanism for partitioning disks than the older Master Boot Record (MBR) partitioning scheme that was common to PCs.

A partition is a contiguous space of storage on a physical or logical disk that functions as if it were a physically separate disk. Partitions are visible to the system firmware and the installed operating systems. Access to a partition is controlled by the system firmware before the system boots the operating system, and then by the operating system after it is started.

> What is wrong with MBR partitioning?
MBR disks support only four partition table entries. If more partitions are wanted, a secondary structure known as an extended partition is necessary. Extended partitions can then be subdivided into one or more logical disks.

By convention, Windows creates MBR disk partitions and logical drives on cylinder boundaries based on the reported geometry, although this information no longer has any relationship to the physicalcharacteristics of the hardware (disk driver or RAID controller). Starting with Windows Vista and Windows Server 2008, more logical boundaries are selected when the hardware provides better hints at the true cache or physical alignment. Because this partition information is stored on the drive itself, the operating system is not dependent on the alignment.

MBR partitioning rules are complex and poorly specified. For example, does cylinder alignment mean that each partition must be at least one cylinder in length. An MBR partition is identified by a two-byte field, and coordination is necessary to avoid collision. IBM originally provided that coordination; today there is no single authoritative list of partition identifiers.

Another common practice is to use partitioned or "hidden" sectors to hold specific information. That practice is undocumented and results in severe system problems that are difficult to debug. Over the years, vendor-specific implementations and tools were released to the public, making support difficult.

> Why do we need GPT?
GPT disks can grow to a very large size. The number of partitions on a GPT disk is not constrained by temporary schemes such as container partitions as defined by the MBR Extended Boot Record (EBR). The GPT disk partition format is well defined and fully self-identifying. Data critical to platform operation is located in partitions and not in unpartitioned or "hidden" sectors. GPT disks use primary and backup partition tables for redundancy and CRC32 fields for improved partition data structure integrity. The GPT partition format uses version number and size fields for future expansion.

Each GPT partition has a unique identification GUID and a partition content type, so no coordination is necessary to prevent partition identifier collision. Each GPT partition has a 36-character Unicode name. This means that any software can present a human-readable name for the partition without any additional understanding of the partition.

> What is the GPT format for basic disks?
Basic disks are the storage types most often used with Windows. The term basic disk refers to a disk that contains partitions, such as primary partitions and logical drives, and these in turn are usually formatted with a file system to become a volume for file storage.

The protective MBR area exists on a GPT partition table for backward compatibility with disk management utilities that operate on MBR. The GPT header defines the range of logical block addresses that are usable by partition entries. The GPT header also defines its location on the disk, its GUID, and a 32-bit cyclic redundancy check (CRC32) checksum that is used to verify the integrity of the GPT header.

Each entry in the GUID partition table begins with a partition type GUID. The 16-byte partition type GUID, which is similar to a System ID in the partition table of an MBR disk, identifies the type of data that the partition contains and identifies how the partition is used, for example, whether it is a basic disk or a dynamic disk. Note that each GUID partition entry has a backup copy.

> What is the GPT format for dynamic disks?
Dynamic disks were first introduced with Windows 2000 and provide features that basic disks do not, such as the ability to create volumes that span multiple disks (spanned and striped volumes) and the ability to create fault-tolerant volumes (mirrored and RAID-5 volumes). Like basic disks, dynamic disks can use the MBR or GPT partition styles on systems that support both. For more information about dynamic disks, see Basic and Dynamic Disks.

> Is UEFI required for a GPT disk?
No. GPT disks are self-identifying. All the information needed to interpret the partitioning scheme of a GPT disk is completely contained in structures in specified locations on the physical media.

> How big can a GPT disk be?
In theory, a GPT disk can be up to 2^64 logical blocks in length. Logical blocks are commonly 512 bytes in size.
The maximum partition (and disk) size is a function of the operating system version. Windows XP and the original release of Windows Server 2003 have a limit of 2TB per physical disk, including all partitions. For Windows Server 2003 SP1, Windows XP x64 edition, and later versions, the maximum raw partition of 18 exabytes can be supported. (Windows file systems currently are limited to 256 terabytes each.)

> How many partitions can a GPT disk have?
The specification allows an almost unlimited number of partitions. However, the Windows implementation restricts this to 128 partitions. The number of partitions is limited by the amount of space reserved for partition entries in the GPT.

> Can a disk be both GPT and MBR?
No. However, all GPT disks contain a Protective MBR.

> What is a Protective MBR?
The Protective MBR, beginning in sector 0, precedes the GPT partition table on the disk. The MBR contains one type 0xEE partition that spans the disk.

> Why does the GPT have a Protective MBR?
The Protective MBR protects GPT disks from previously released MBR disk tools such as Microsoft MS-DOS FDISK or Microsoft Windows NT Disk Administrator. These tools are not aware of GPT and do not know how to properly access a GPT disk. Legacy software that does not know about GPT interprets only the Protected MBR when it accesses a GPT disk. These tools will view a GPT disk as having a single encompassing (possibly unrecognized) partition by interpreting the Protected MBR, rather than mistaking the disk for one that is unpartitioned.

> Why would a GPT-partitioned disk appear to have an MBR on it?
If this occurred, you must have used an MBR-only-aware disk tool to access the GPT disk.

> Can a disk be both GPT and MBR?
> What is a Protective MBR?
> Why does the GPT have a Protective MBR?

> Can Windows XP x64 read, write, and boot from GPT disks?
Windows XP x64 Edition can use GPT disks for data only.

> Can the 32-bit version of Windows XP read, write, and boot from GPT disks?
The 32-bit version will see only the Protective MBR. The EE partition will not be mounted or otherwise exposed to application software.

> Can the 32- and 64-bit versions of Windows Server 2003 read, write, and boot from GPT disks?
Starting with Windows Server 2003 Service Pack 1, all versions of Windows Server can use GPT partitioned disks for data. Booting is only supported for 64-bit editions on Itanium-based systems.

> Can Windows 7, Windows Vista, and Windows Server 2008 read, write, and boot from GPT disks?
Yes, all versions can use GPT partitioned disks for data. Booting is only supported for 64-bit editions on UEFI-based systems.

> Can Windows 2000, Windows NT 4, or Windows 95/98 read, write, and boot from GPT?
No. Again, legacy software will see only the Protective MBR.


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