Using Fault-Tolerant Disk Arrays


Windows Server 2003 supports both hardware- and software-based RAID volumes to create fault tolerance for disk failures. Redundant Array of Inexpensive Disks (RAID) provides different levels of configuration that deliver disk fault tolerance, and formatting such volumes using the NT File System (NTFS) also allows directory- and file-based security, data compression, and data encryption to be enabled. Hardware-based RAID is preferred because the disk management tasks are offloaded to the RAID controller, reducing the load on the operating system. When a disk is available to Windows Server 2003, it can be configured as a basic disk or a dynamic disk.

Disk Types

Windows Server 2003 can access disks connected directly to the server from an IDE controller, SCSI controller, or an external RAID controller. RAID disks can provide faster disk access times but also can provide fault tolerance for disk failures.

Hardware-based RAID is achieved when a separate RAID disk controller is used to configure and manage the RAID array. The RAID controller stores the information on the array configuration, including disk membership and status. Hardware-based RAID is preferred over Windows Server 2003 software-based RAID because the disk management processing is offloaded to the RAID card, reducing processor utilization.

As mentioned previously, Windows Server 2003 supports two types of disks: basic and dynamic. Basic disks are backward compatible, meaning that basic partitions can be accessed by previous Microsoft operating systems such as MS-DOS and Windows 95 when formatted using FAT; and when formatted using NTFS, Windows NT, Windows 2000, and Windows Server 2003 can access them. Dynamic disks are managed by the operating system and provide several configuration options, including software-based RAID sets and the ability to extend volumes across multiple disks.

Basic Disks

Basic disks can be accessed by Microsoft Windows Server 2003 and all previous Microsoft Windows or MS-DOS operating systems. These disks can be segmented into as many as four partitions. The combination of partitions can include up to four primary partitions or three primary partitions and one extended partition. Primary partitions can be used to start legacy operating systems and are treated as a single volume. An extended partition can be broken into multiple logical drives. Each logical drive is managed as a separate volume, allowing administrators to create as many volumes on a basic disk as necessary. Basic partitions and logical drives can be formatted as either FAT, FAT32, or NTFS disks. Basic partitions are also referred to as basic volumes.

Dynamic Disks

Dynamic disks can be segmented into several logical drives referred to as dynamic volumes. Dynamic disks are managed by the operating system using the Virtual Disk Service (VDS). Many volumes can be defined on a dynamic disk, but limiting the number of volumes to 32 or fewer is recommended. After a disk is converted to a dynamic disk, it can be mounted only by Windows Server 2003 systems, but the data can still be accessed by other operating systems using Windows Server 2003 file services, including Web services, FTP services, file shares, and other client/server-based applications.

In some configurations, dynamic volumes can span two or more disks and provide disk fault tolerance. Dynamic volume types provided in Windows Server 2003 include the following:

  • Simple volume A simple volume is similar to a basic partition in that the entire volume is treated as a single drive and it does not span multiple disks.

  • Spanned volume A spanned volume is treated as a single drive, but the volume spans two or more disks. Spanned volumes provide no disk fault tolerance but can be used to meet disk storage needs that exceed the capacity of a single disk. 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 partition needs to be extended to add disk space. For instance, if an application 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 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. The allocated space on each of the disks can be of different sizes.

  • Striped volume A striped volume or RAID 0compatible volume requires two or more 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 data would be inaccessible. Stripe sets require the exact amount of disk space on each of the allocated disks. For example, to create a 4GB stripe set array with two disks, 2GB of unallocated space would be required on each disk.

  • RAID 5 volume Software-based RAID 5 volumes require three or more disks and provide faster read/write disk access than a single disk. The space or volume provided on each disk of the RAID set must be equal. RAID 5 sets can withstand a single disk failure and can continue to provide access to data using only the remaining disks. 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 or to continue to provide data access. RAID 5 parity information requires the space of a single disk in the array or can be computed using the formula

    (N1)*S = T

    where N is the number of disks, S is the size of the allocated space on each disk, and T is the total available space for storage. For example, if five disks allocate 10GB each for a RAID 5 array, the total available disk space available for storage will be (51)*10GB = 40GB. The 10GB are reserved for parity information.

  • Mirrored volume Mirrored or RAID 1compatible volumes require two separate disks, and the space allocated on each disk must be equal. Mirrored sets duplicate data across both disks and can withstand a single disk failure. Because the mirrored volume is an exact replica of the first disk, the space capacity of a mirrored set is limited to half of the total allocated disk space.

Tip

As a best practice, try to provide disk fault tolerance for your operating system and data drives, preferably using hardware-based RAID sets.


For the rest of this chapter, both basic partitions and dynamic volumes will be referred to as volumes.

Disk Formatting

Windows Server 2003 supports formatting basic and dynamic volumes using the NTFS, FAT, or FAT32 file system. FAT volumes are supported by MS-DOS and all Microsoft Windows operating systems, but should be limited to 2GB if MS-DOS access is necessary. FAT32 was first supported by Microsoft with Windows 95, but these partitions cannot be read by MS-DOS, Windows for Workgroups, or Windows NT. Windows Server 2003 NTFS volumes are supported by Windows NT 4.0 with Service Pack 6a or higher and all versions of Windows 2000, Windows XP, and Windows Server 2003. File shares can be created on each type of disk format, but NTFS volumes provide extended features such as volume storage quotas, shadow copies, data compression, file- and folder-level security, and encryption.

Managing Disks

Disks in Windows Server 2003 can be managed using a variety of tools included with the operating system. Disk tasks can be performed using the Disk Management Microsoft Management Console (MMC) snap-in from a local or remote server console or using a command-line utility called diskpart.exe.

Using the Disk Management MMC Snap-in

Most disk-related administrative tasks can be performed using the Disk Management MMC snap-in. This tool is located in the Computer Management console, but the stand-alone snap-in can also be added in a separate Microsoft Management Console window. Disk Management is used to identify disks, define disk volumes, and format the volumes. Starting in Windows Server 2003, the Disk Management console can be used to manage disks on remote machines. If a disk is partitioned and formatted during the Windows Server 2003 setup process, when installation is complete, the disk will be identified as a basic disk. After Windows Server 2003 is loaded and disk management can be accessed, this disk can be converted to a dynamic disk, giving server administrators more disk configuration options.

Using the Diskpart.exe Command-Line Utility

Diskpart.exe is a functional and flexible command-line disk management utility. Most disk tasks that can be performed using the Disk Management console can also be performed using this command-line utility. Using diskpart.exe, both basic volumes and dynamic volumes can be extended, but Disk Management can extend only dynamic volumes. Diskpart.exe can be run with a script to automate volume management.

As a sample of scripting diskpart.exe, using a filename like c:\drive info.txt, the following information can be used to extend a volume using unallocated space on the same disk:

Select Volume 2 Extend Exit 


When you're creating the command script file, be sure to press Enter at the end of each command so that when the script is called out, the Enter keystroke is executed.

At the command prompt, run

Diskpart.exe /s c:\drive_info.txt


Now volume 2 will be extended with all the remaining unallocated disk space on the same disk.

Note

If you want to extend a basic volume using diskpart.exe, the unallocated disk space must be on the same disk as the original volume and must be contiguous with the volume you are extending. Otherwise, the command will fail.


Creating Fault-Tolerant Volumes

Windows Server 2003 supports fault-tolerant disk arrays configured and managed on a RAID disk controller or configured within the operating system using dynamic disks. To create arrays using a RAID controller, refer to the manufacturer's documentation and use the appropriate disk utilities. Software-based RAID can be configured using the Disk Management console or the command-line utility diskpart.exe.

Converting Basic Disks to Dynamic Disks

Before an administrator can create software-based fault-tolerant volumes, the necessary disk must be converted to a dynamic disk. To convert a basic disk to a dynamic disk, follow these steps:

1.

Log on to the desired server using an account with Local Administrator access.

2.

Click Start, All Programs, Administrative Tools, Computer Management.

3.

In the left pane, if it is not already expanded, double-click Computer Management (local).

4.

Click the plus sign next to Storage.

5.

Select Disk Management.

6.

In the right pane, verify that the disk containing the system volume is marked as dynamic.

7.

If each of the necessary disks is already dynamic, close Computer Management by selecting File, Exit.

8.

If the drive is marked as basic, right-click the drive and select Convert to Dynamic Disk. Select the appropriate disk, press OK, verify the information in the dialog box, and then click Convert.

9.

Repeat the preceding steps for each disk that will participate in a spanned, mirrored, striped, or RAID 5 volume.

10.

If the disk containing the system drive is converted, the operating system may request multiple system reboots to first unmount the drive and then to convert it to a dynamic disk. After you restart, the disk will be recognized as a new disk, and another reboot will be necessary. Reboot the system as requested.

11.

After all necessary disks are converted to dynamic, use Disk Management in the Computer Management console to verify that the conversion was successful and the disks can still be accessed.

Creating Fault-Tolerant Disk Volumes Using Dynamic Disks

Creating a fault-tolerant disk volume in Windows Server 2003 requires having two disks available for a mirrored volume and at least three disks for a RAID 5 volume. To create a mirrored system volume, follow these steps:

1.

Log on to the desired server using an account with Local Administrator access.

2.

Click Start, All Programs, Administrative Tools, Computer Management.

3.

In the left pane, if it is not already expanded, double-click Computer Management (local).

4.

Click the plus sign next to Storage.

5.

Select Disk Management.

6.

In the right pane, right-click the system volume and choose Add Mirror.

7.

If more than one additional dynamic disk is available, choose the disk on which to create the mirror for the system volume and click Add Mirror.

8.

The volumes on each disk start a synchronization process that may take a few minutes or longer, depending on the size of the system volume and the types of disks being used. When the mirrored volume's status changes from Resynching to Healthy, select File, Exit in the Computer Management console to close the window.

9.

Log off the server console.

A Windows Server 2003 RAID 5 volume requires three separate dynamic disks, each containing an equal amount of unallocated disk space for the volume. To create a RAID 5 volume using Disk Management, follow these steps:

1.

Log on to the desired server using an account with Local Administrator access.

2.

Click Start, All Programs, Administrative Tools, Computer Management.

3.

In the left pane, if it is not already expanded, double-click Computer Management (local).

4.

Click the plus sign next to Storage.

5.

Select Disk Management.

6.

Right-click Disk Management and select New, Volume.

7.

Click Next on the New Volume Wizard Welcome screen.

8.

On the Select Volume Type page, select the RAID 5 radio button and click Next to continue.

9.

On the Select Disks page, select a disk that will participate in the RAID 5 volume from the Available pane and click the Add button.

10.

Repeat the preceding steps for the two or more remaining disks until all the participating disks are in the Selected pane.

11.

After all the disks are in the Selected pane, the maximum available volume size is automatically calculated, as displayed in Figure 30.1. Click Next to continue, or enter the correct size in megabytes and then click Next.

Figure 30.1. Configuring the RAID 5 volume's storage capacity.


12.

On the Assign Drive Letter or Path page, choose the drive letter to assign this volume. Other options include not assigning a drive letter to the volume and mounting the volume in an empty NTFS folder in a separate volume. Choose the option that meets your requirements and click Next to continue.

13.

On the Format Volume page, choose whether to format the volume and enable data compression. Click Next to continue.

Tip

When you're formatting RAID 5 volumes, perform a complete format to avoid loss of disk performance later when data is first copied to the volume.

14.

Click Finish on the Completing the New Volume Wizard page to create the volume and start the format.

15.

The volume is then formatted, which can take a few minutes. When the formatting starts, you can close the Computer Management console and log off the server.

16.

When prompted to restart your server, choose whether you want to restart the system now by selecting Yes or restart the system at a different time by selecting No.

Tip

Before you start using the volume, you should check it for health using the Disk Management MMC snap-in.





Microsoft Windows Server 2003 Unleashed(c) R2 Edition
Microsoft Windows Server 2003 Unleashed (R2 Edition)
ISBN: 0672328984
EAN: 2147483647
Year: 2006
Pages: 499

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