Windows Server 2008 R2 Unleashed (217 page)

system platforms and third-party disk management tools.

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GUID Partition Table (GPT) Disks

GPT disks were first introduced in Windows with Windows Server 2003 Service Pack 1.

GPT disks are recommended for disks that exceed 2TB in size. GPT disks can support an

unlimited number of primary partitions and this can be very useful when administrators

are leveraging large external disk arrays and need to segment data for security, hosting, or

distributed management and access. GPT disks are only recognized by Windows Server

2003 SP1 and later Windows operating systems. Attempting to manage a GPT disk using a

previous operating system or third-party MBR disk management tool will be blocked and

virtually inaccessible.

Basic Disk

A Windows disk is defined as a basic or a dynamic disk regardless of whether the disk is an

MBR or a GPT disk. A basic disk supports only simple volumes or volumes that exist on a

single disk and partition within Windows. Basic disks contain no fault tolerance managed

by the Windows operating system, but can be fault tolerant if the disk presented to

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File System Management and Fault Tolerance

Windows is managed by an external disk controller and is configured in a fault-tolerant

array of disks.

Basic disks are easier to move across different operating systems and usually are more

compatible with Windows and third-party disk and file system services and management

tools. Basic disks also support booting to different operating systems stored in separate

partitions. Furthermore, and most important, if the disk presented to Windows is from a

SAN that includes multiple paths to the disk, using a basic disk will provide the most reli-

able operation as a different path to the disk might not be recognized if the disk is defined

within Windows as a dynamic disk.

Dynamic Disk

Dynamic disks extend Windows disk functionality when managing multiple disks using

Windows Server 2008 R2 is required. Windows administrators can configure dynamic disks

to host volumes that span multiple partitions and disks within a single system. This allows

administrators to build fault-tolerant and better performing volumes when RAID

controllers are not available or when a number of smaller disks need to be grouped

together to form a larger disk.

In some server deployments, dynamic disks are required as the disk controllers do not

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support the necessary performance, fault-tolerance, or volume size requirements to meet

the recommended system specifications. In these cases, dynamic disks can be used to

create larger volumes, fault-tolerant volumes, or volumes that can read and write data

across multiple physical disks to achieve higher performance and higher reliability.

Dynamic disks are managed by the operating system using the Virtual Disk Service (VDS).

Virtual Hard Disks

Virtual hard disks or VHDs are used by virtual machines to emulate Windows disks.

Virtual hard disks can be created on an existing Windows Server 2008 R2 system using the

Hyper-V Management console or they can be created directly using the Disk Management

console. VHDs are primarily created on the Windows host system as a file on an existing

Windows volume that has a .vhd extension. VHD disks can be created to be fixed size or

dynamically expanding. A fixed-sized VHD that is 10GB in size will equate to a 10GB file

on the Windows host server volume. A dynamically expanding VHD file will expand as

files are stored on it, only as necessary. VHD files can easily be moved across servers and

between virtual machines, and also can be expanded quite easily, granted that the VHD is

not in use and there is ample free space on the host volume. VHD files can be attached

directly to a Windows Server 2008 R2 host using the Disk Management console, unlike in

previous releases, which required scripts to mount the file. This added functionality is a

needed improvement to the integrated VSS Hyper-V backup functionality, included with

Windows Server Backup and available to third-party backup software vendors. Creating

and attaching a VHD file to a Windows Server 2008 R2 host is detailed later in this

chapter, but for more information on VHD files and their management, refer to Chapter

37, “Deploying and Using Windows Virtualization.”

Windows Server 2008 R2 Disks

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Partition or Volume

When referring to Windows disks, administrators might consider partitions and volumes

interchangeable. In fact, even though the graphical user interface makes no clear distinc-

tion and might refer to everything as a volume, volumes only exist on dynamic disks and

partitions only exist on basic disks. This is especially important when managing disks

using the diskpart.exe command-line utility, which defines a clear delineation between

partitions and volumes.

Mount Point

When a new volume is created in Windows, it can be assigned a drive letter or mounted

into an existing empty folder on an existing volume. When a volume is mounted into a

folder, this is known as a mount point or junction point. Mount points can be very useful

in situations where administrators want to simplify disk access for end users, but must also

make use of a number of small disks versus a single large disk. For example, on a database

server with three disks, an administrator might assign disk1 the D drive, disk2 would be

mounted in d:\data, and disk3 would be mounted in d:\logfiles. Any administrator would

only need to connect to the D drive to access the databases or log files. One thing that

administrators must test before using mount points is to see that all clients, applications,

and backup agents support the use of mount or junction points and can successfully

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access and back up data stored within them. With many backup applications, enabling a

backup job to back up data stored on a mounted volume is not the default and can cause

major problems if the correct backup configuration is not selected before a failure occurs.

Simple Volumes

A simple volume is a single partition created on a single basic or dynamic disk. On a basic

disk, simple volumes can be extended to include free, unallocated space that exists in a

sequential section of the disk. To extend a simple volume to a noncontiguous, unallo-

cated space on the same disk or a different disk, the disk will need to be upgraded to a

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dynamic disk.

Spanned Volumes

A spanned volume is treated as a single drive, but the volume spans two or more disks or

different noncontiguous areas of the same disk. Spanned volumes provide no disk fault

tolerance but can be used to meet disk storage needs that exceed the capacity of a single

disk or volume. Spanned volumes are slowest when it comes to reading and writing data

and are recommended only when the space of more than a single disk is necessary or an

existing simple volume needs to be extended to add disk space and there is no available,

unallocated space located next to the volume. For instance, if an application, file share, or

service is dependent on the drive letter, does not support the moving of data or system

files to another drive, and the current drive is nearly full, a simple volume can be

upgraded to a spanned volume and extended with unallocated space on the same or

another disk to add additional disk space. A simple volume that has been extended with

unallocated space on the same disk is still considered a simple volume. If the simple

volume is extended to a different disk, it is automatically converted to a spanned volume.

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File System Management and Fault Tolerance

The allocated space on each of the disks can be different sizes, and there is no space lost

when creating a spanned volume. One thing to keep in mind, though, is that a spanned

volume can never be reverted to a simple volume.

Striped Volumes

A striped volume or RAID-0 compatible volume requires two or more Windows dynamic

disks and provides the fastest of all disk configurations. Striped volumes read and write

data from each of the disks simultaneously, which improves disk access time. Striped

volumes utilize all the space allocated for data storage but provide no disk fault tolerance.

If one of the disks should fail, the entire data set would become inaccessible. Stripe sets

require the exact amount of disk space on each of the allocated disks. For example, to

create a 15GB stripe set array with three disks, 5GB of unallocated space would be required

on each disk.

Fault-Tolerant Volumes

When fault-tolerant disk arrays managed by hardware controllers are not available, fault-

tolerant volumes can be created using multiple Windows dynamic disks. Fault-tolerant

volumes in Windows are able to maintain data availability in the event of a single disk

failure. Windows Server 2008 R2 supports two types of fault-tolerant volumes, including

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mirrored volumes and RAID-5 volumes.

Mirrored Volumes

Mirrored or RAID-1 compatible volumes require two separate disks to create. Furthermore,

the size of the volume must be equal and available in one contiguous, unallocated section

of each of the disks. Mirrored volumes duplicate data across each disk and can withstand

the failure of a single disk. Because the mirrored volume is an exact replica of the first

disk, the total space capacity is the capacity of one disk.

RAID-5 Volumes

Software-based RAID-5 volumes require three or more Windows dynamic disks and can

provide faster disk read access than a single disk because all disks in the set can be read at

the same time. Write performance can be slower than a single disk because of the parity

stripe that must be generated and written. The space allocated to the RAID-5 volume on

each disk in the volume must be equal and contiguous unallocated space. For example, to

create a RAID-5 volume that requires 100GB on each disk, a disk with two separate areas

of 50GB of unallocated space cannot be used to participate in the volume.

RAID-5 sets can withstand the failure of a single disk in the volume. During a disk failure,

the remaining disks in the volume will continue to provide access to data but at a slower

or degraded rate. This capability is achieved by reserving a small portion of each disk’s

allocated space to store data parity information that can be used to rebuild a failed disk

and to continue to provide data access. This is called a parity stripe. RAID-5 parity infor-

mation requires the total space of a single disk in the array. For example, if five 10GB

dynamic disks are used to create a single RAID-5 volume, 40GB would be available for data

Managing Windows Server 2008 R2 Disks

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storage. The reserved 10GB would be spread evenly across all five disks. The formula for

usable capacity of a RAID-5 array is (N - 1) * S, where N is the total number of drives in

the array and S is the capacity of the smallest drive in the array.

Utilizing External Disk Subsystems

Windows Server 2008 R2 is capable of utilizing and, in some cases, managing disks stored

in external storage area networks (SANs) or disk subsystems. Many enterprise organiza-

tions, and actually many midsize organizations, deploy Windows systems connected to

external disk storage. As organizations move toward consolidating and virtualizing servers,

the need to provide fault-tolerant disk storage at the organization level instead of at the

server level has become more common.

Hardware-Based Disk Arrays

As a best practice, whenever possible, use RAID-compatible disk controllers or SANs to

provide fault-tolerant disk volumes to Windows Server 2008 R2 systems. Using externally

managed RAID arrays will reduce server processing requirements and can also improve the

recoverability of the system if operating system corruption is encountered.

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Boot from Storage Area Networks

Many SAN vendors and Microsoft tout the ability to deploy diskless servers that boot from

SAN disks. This configuration allows SAN vendors to completely manage the Windows

system disks and can simplify the recovery or replacement of a server based on hardware

failure or scheduled replacement.

Managing External Storage

Windows Server 2008 R2 provides a tool called “Storage Manager for SANs” to assist

administrators in managing SAN-based disks. In most cases, however, SAN vendors

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provide and recommend the use of their own vendor-based management tools.

External Storage Support Requirements

Microsoft has very tight specifications when it comes to supporting external storage for

Windows servers. Administrators who plan to utilize external storage should review the

Windows Server 2008 R2 Hardware Compatibility List for external storage devices, storage

controllers, driver types, driver versions, and controller firmware versions to verify

supported compatibility before purchasing any external storage devices—that is, if the

administrators want to be supported by Microsoft in the event of disk or data corruption.

Managing Windows Server 2008 R2 Disks

Disks in Windows Server 2008 R2 can be managed using a few different tools included