Inside Windows Server 1002003 [Electronic resources] نسخه متنی

Working with GPT Disks

As I mentioned at the start of this chapter, the Itanium platform uses a new disk partitioning scheme called GUID Partition Tables, or GPTs. This new scheme was developed to address several shortcomings of the classic MBR approach:

• An MBR disk can have only four primary partitions.




• The maximum partition size is limited by a truncated Cylinder/Head/Sector geometry notation.




• There is no coordination between the various partition schemes. This causes a problem for disk utility vendors who must anticipate how a particular scheme will align with cylinder boundaries for both bare disks and virtual disks presented by RAID controllers.




• MBR disks have spawned a generation of utilities and operating systems that use hidden sectors containing critical storage parameters and components.




• A disk holds a single copy of the MBR, making it a single point of failure if the sector should go bad.

Chapter 1, "Installing and Configuring Windows Server 2003," tells how to use Setup to initially partition a GPT disk for Windows Server 2003, and Chapter 3, "Adding Hardware," contains the details for installing and configuring new GPT disks. This topic contains general information that compares GPT disk operation to MBR disks.

GPT Improvements

GPT disks address the classic MBR limitations by replacing the system entirely. A GPT disk does have a classic MBR on the first sector, but it is there purely as a protective measure. Standard disk utilities would interpret the lack of an MBR as a lack of partitioning and would possibly automatically repartition the drive, causing a loss of the GPT-based data.

In place of the classic MBR, a GPT disk uses a database to track partition identities and locations. The database identifies specific partition types by a Globally Unique Identifier, or GUID, hence the name. A vendor can define a new partition type by creating a GUID and be confident that the number has not been preempted by another vendor.

GPT disks have the following advantages over MBR disks:

• Partitions start at sector boundaries, not cylinder boundaries. This greatly simplifies interoperability between drives in the same machine.




• GPT disks can have 128 partitions.




• A single partition can be up to 18 exabytes, equivalent to having just under 19 million one-terabyte disk arrays connected together.




• Each GPT partition entry is copied for redundancy. In addition, a Cyclic Redundancy Check (CRC) value is stored along with the partition data to check for corruption.




• All data is stored in defined partitions. The operating system hides partitions used for purposes other than storing user data.

You can have MBR disks on an Itanium system, but you must boot to a GPT disk. The Extensible Firmware Interface (EFI) on an Itanium system expects to find OS loader information on the boot drive. Allowance was made in the EFI specification to permit storing this information on an MBR disk, but Windows Server 2003 and XP will only boot from an EFI disk.

You can convert an MBR disk to a GPT disk and vice-versa, but you must remove all existing partitioning, which causes a loss of data if you do not have a backup. Note that this operation is

not the same as converting a basic disk to a dynamic disk. GPT partitions are not contained in the Logical Disk Manager database until the disk is converted to a dynamic disk.

GPT Format

Figure 14.12 shows the default partitions on a GPT boot disk. Here is a description of each element.

EFI System Partition (ESP)

The Chapter 3, "Adding Hardware," has information on installing and formatting a GPT disk. Here are some points to remember about the ESP:

• If the ESP is created by Windows Server 2003 Setup, the ESP is given one percent of the disk size up to 100MB. The one-percent value is not recalculated if additional storage is added via hardware RAID.




• The ESP cannot be mirrored, striped, extended, or spanned.




• The ESP should be the first partition so that it does not interfere with converting the disk to a dynamic disk. A GPT disk cannot be converted to a dynamic disk if the EFS is nestled between two data partitions.




• System vendors are encouraged to place their diagnostic and value-add files in their own partition, called an

  OEM partition , and not in the ESP. Microsoft is not a system vendor and places disk utilities in an MSUTIL folder at the root of the ESP.




• The ESP is formatted as FAT. Unlike other FAT-formatted partitions, the ESP is not given a drive letter by Windows and does not appear in the Disk Management console. However, you can view the contents of the ESP using the MOUNTVOL utility with the following command: mountvol <

  drive_letter> /s.

Microsoft Reserved Partition (MSR)

This partition sets aside space for Microsoft to use for specialized structures. For example, the Logical Disk Manager (LDM) database is put into the MSR. Other operating system vendors can create similar partitions. Here are the key points concerning MSR partitions:

• Like the ESP, the MSR should be created ahead of data partitions so that the disk can be converted to a dynamic disk, if desired. On a disk partitioned by Windows Server 2003 Setup, the MSR will be the second partition.




• The MSR is 32MB for disks less than 16GB and 128MB for [ge]16GB.




• The MSR does not use a recognizable file system. It contains data structures only understood by Windows Server 2003 system services.




• Users and administrators cannot view the MSR.

OEM Specific Partitions

A hardware vendor can create a partition to hold diagnostic and setup applications. This is similar to how Compaq handles their Smart Start suite of utilities.

The vendor is responsible for generating the GUID for its partitions and for creating and formatting its OEM partition on a GPT disk prior to shipment. Ordinarily, users and administrators cannot view the contents of an OEM partition. It is only accessible via tools supplied by the vendor.

Data Partitions

This partition type contains user data, which includes the operating system files. You can create additional data partitions. All data partitions must be contiguous for a GPT disk to be converted to a dynamic disk.

Microsoft data partitions can be formatted in Windows Server 2003 using any of the three supported formatting options: FAT, FAT32, or NTFS.

If you have operating systems other than Windows installed in other partitions, Windows might not be able to recognize the file systems. Partitions containing recognizable file systems are assigned drive letters.

GPT Requirements

The following restrictions apply when managing GPT disks:

• Only the IA64 version of Windows Server 2003 can read GPT disks.




• GPT disks can only be created on fixed spinning storage. You must use MBR partitioning for detachable disk drives and removable platter drives. For this reason, you cannot boot an Itanium machine to a Firewire drive as you can with an IA32 machine.




• You cannot use disk-imaging programs on GPT disks. This would result in a clone of the GUID. An OEM Preinstallation Kit (OPK) supplied by Microsoft supports cloning by zeroing out the GUIDs then initializing them during a mini-setup when the machine is first started.




• The boot disk for an IA64 system must be a GPT disk. Other disks in an IA64 system can be MBR or GPT.




• An MBR disk cannot be upgraded to a GPT disk. You must delete all legacy MBR partitions, then create a new GPT partition table.

Working with GPT Disks

Just like an IA32 system, the primary Windows Server 2003 tool for accessing and managing a GPT disk is the Disk Management console. The console is coded to create GPT partitions on a GPT disk and MBR partitions on an MBR disk. The Disk Management console on an IA32 system cannot be used to manage GPT disks across the networkChapter 3, "Adding Hardware," provides the steps.

The big advantage of having a GPT disk over having an MBR disk is the partitioning flexibility afforded by the GPT partition table. You can create up to 128 partitions on a GPT basic disk. Figure 14.13 shows the Disk Management console for a basic GPT disk with over 40 partitions.

If you want to have more sophisticated configurations with striping, RAID 5, or mirroring, you must convert the GPT basic disk to a dynamic disk. This replaces the GPT partition table with an LDM database stored in the Microsoft Reserved Partition. The LDM service operates the same on an IA64 system as it does on an IA32 system. The same fault tolerant configurations are permitted, and the same procedure is used to transfer disks between systems. The only LDM restrictions unique to a GPT disk are as follows:

• All basic data partitions must be contiguous.




• A GPT boot disk cannot be mirrored to an MBR disk.




• The EFI System Partition (ESP) cannot be mirrored.