Adding New GPT and MBR Disks to Windows Server 2008 Systems
The purpose of this chapter of Windows Server 2008 Essentials is to provide an overview of the decisions and steps necessary to add new disk drives to a Windows Server 2008 system. Installing a new disk drive is not just about formatting the drive and creating volumes. With Windows Server 2008 decisions need to be made about the partitioning style since both Master Boot Record (MBR) or GUID Partition Table (GPT) disks are supported in this latest version of Windows. Given this requirement, this chapter will also provide an explanation of the difference between MBR and GPT disks.
An Overview of MBR and GPT Partitions Styles
Before a disk can be used to store data it must first have partitions created on it. These partitions are then formated with a particular file system (FAT, FAT32, NTFS) and used either as a basic disk or as dynamic disk). Obviosuly, the operating system needs some mechanism for organize and manage these disk partitions and Windows Server 2008 provides two such partitioning styles known as Master Boot Record (MBR) or GUID Partition Table (GPT).
The MBR style was originally developed for x86-based computer systems is by far the most common style in use at present. GPT, on the other hand was originally developed for 64-bit Itanium based systems. With the arrival of Windows Server 2008 both x86 32-bit and 64-bit systems support MBR and GPT partition styles. It is important that 32-bit systems can only boots from MBR disk partitions and 64-bit systems can only boot from GPT partitions.
Understanding MBR Disks
With MBR the first sector of the disk is reserved to store a partition table and the master boot record. The remainder of the disk is divided into partitions, information about which is stored in the partition table. MBR supports volume sizes up to 4TB.
MBR on a basic disk supports two partition types, primary and extended. The primary partition has a file system created directly on it and is then either assigned a drive letter or mount point by which it is referenced by the user. An extended partition is divided into one or more logical drives, each of which is then formatted and then assigned drive letter or mount point.
A basic disk with MBR partition style can support either four primary partitions or three primary partitions and one extended partition (which in turn can support multiple logical volumes).
Understanding GPT Disks
GPT disks differ quite considerably from MBR disks. In terms of capability, GPT supports disks up to 18 Exabytes in size with up 128 partitions. GPT also differs from MBR in terms of layout.
At the start of a GPT disk is an MBR. The MBR in this case, however, is provided purely so that the disk will be recognized by disk utilities that do not recognize GPT disks. To an MBR disk utility, the disk will appear to be an MBR disk with a single partition taking up the entire disk space.
Immediately after the MBR is the primary GPT header which defines the blocks on the disk available for partitions and information about the number and respective sizes of any partitions on the disk. The GPT header also includes information about its own location on the disk drive and a pointer to a backup GPT header located in the final sectors of the drive. The backup GPT header is used in the event that the primary header becomes corrupted. Finally, the GPT header contains a CRC32 checksum of itself (including the partition table) so that the system firmware can verify the integrity of the header information before accessing the data on the disk. If the checksum fails the firmware switches to the backup GPT header. If the checksum on backup GPT header also fails the disk is unusable.
In between the primary GPT header at the start of the disk and backup GPT in the final sectors of the disk are the primary partitions. In addition to any data, each partition on a GPT disk has a header containing information about the partition type, the begin and end blocks of the partition and a unique partition GUID.
There are a number of additional partitions often required on a GPT disk. These are the EFI system partition (ESP) and the Microsoft Reserved Partition (MSR). The ESP must be present on the first disk in a system and is required to boot the operating system. The ESP is not mandatory on other disks. When the 64-bit version of Windows Server 2008 is installed both the ESP and MSR are created by the setup process.
One important point of note regarding GPT is that it is not supported on removable disks such as USB and Firewire connected storage devices or disks attached to storage clusters.
