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Achieving Fault Tolerance by Using RAID

Updated: March 28, 2003

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

RAID is commonly implemented for both performance and fault tolerance. With RAID, you can choose to assemble disks to provide fault tolerance, performance, or both, depending on the RAID level that you configure. Table 1.4 summarizes commonly available RAID levels.

Table 1.4   RAID Comparison

 

RAID Level Description Minimum Disks Required Effective Capacity

0

Disk Striping. Two or more disks appear to the operating system as a single disk. Data is striped across each disk during read/write operations. Potentially increases disk access speeds 2X or better. Not fault tolerant.

2

S*N

N = # of disks in array

S = Size of smallest disk in array

1

Disk mirroring. Data is mirrored on two or more disks. Provides fault tolerance, but at a higher cost (space required is double the amount of data). Read performance is increased as well.

2

S

S = Size of smallest disk in array

0+1

Combines RAID 0 and RAID 1; offers the performance of RAID 0 and the protection of RAID 1.

4

S*N/M

N = # of disks in array

S = Size of smallest disk in array

M = Number of mirror sets

5

Disk striping with parity. Provides slower performance than RAID 0, but provides fault tolerance. A single disk can be lost without any data loss. Parity bits are distributed across all disks in the array.

3

S*(N-1)

N = # of disks in array

S = Size of smallest disk in array

From a design perspective, your choice of a RAID solution should be dictated by the type of data being stored. Although RAID 0 offers the fastest read and write performance, it does not offer any fault tolerance, so that if a single disk in a RAID 0 array is lost, all data is lost and will need to be recovered from backup. This might be a good choice for high performance workstations, but might not be suited to mission-critical servers.

RAID 1 allows you to configure two or more disks to mirror each other. This configuration produces slow writes, but relatively quick reads, and provides a means to maintain high data availability on servers, because a single disk can be lost without any loss of data. When more than two disks make up the mirror, the RAID 1 array can lose multiple disks so long as a complete mirrored pair is not lost. When planning a RAID 1 solution, remember that the amount of physical disk space required is twice the space required to store the data.

RAID 0+1 combines the performance benefit of striping with the fault tolerance of mirroring. Compared to RAID 0, writes are slower, but reads are equally fast. Compared to RAID 1, RAID 0+1 offers faster writes and reads but also requires additional storage to create the mirrored stripe sets. This configuration is often ideal for mission-critical database storage, because it offers both fast read access and fault tolerance.

RAID 5 provides fault tolerance: you can lose a single disk in an array with no loss of data. However, RAID 5 operates much more slowly than RAID 0 because a parity bit must be calculated for all write operations. RAID-5 volumes are well suited for reads and also work well in the following situations:

  • In large query or database mining applications where reads occur much more often than writes. Performance degrades as the percentage of write operations increases. Database applications that read randomly work well with the built-in load balancing of a RAID 5 volume.

  • Where a high degree of fault tolerance is required without the cost of the additional disk space needed for a RAID 1 volume. A RAID 5 volume is significantly more efficient than a mirrored volume when larger numbers of disks are used. The space required for storing the parity information is equivalent to 1/Number of disks, so a 10-disk array uses 1/10 of its capacity for parity information. The disk space that is used for parity decreases as the number of disks in the array increases.

Choosing Between Hardware and Software RAID

An additional consideration with RAID implementations is the choice between hardware-based and software-based RAID. With hardware RAID, a hardware RAID controller allows you to configure the RAID level of attached disks. With software RAID, the operating system manages the RAID configuration, along with data reads and writes.

Windows Server 2003 supports the following software RAID types:

  • RAID 0: Up to 32 disks striped

  • RAID 1: Two disks mirrored

  • RAID 5: Up to 32 disks striped with parity

To configure software RAID, a disk must be configured as a dynamic disk. Although software RAID has lower performance than hardware RAID, software RAID is inexpensive and easy to configure because it has no special hardware requirements other than multiple disks. If cost is more important than performance, software RAID is appropriate. If you plan to use software RAID for write-heavy workloads, use RAID-1 instead of RAID-5. Using software-based RAID with Windows Server 2003 dynamic disks is a good choice for providing fault tolerance using SCSI or EIDE disks to non-mission-critical servers that require fault tolerance and can accommodate the added CPU load imposed by software RAID. This solution is ideal for small to medium organizations that need to add a level of fault tolerance while avoiding the cost of hardware RAID controllers.

For more information about software RAID and disk management, see Storage Technologies Collection (http://go.microsoft.com/fwlink/?linkid=4681).

Using Dynamic Disks with Hardware RAID

You can also use dynamic disks with hardware-based RAID solutions. Using dynamic disks with hardware RAID can be useful in the following situations:

  • You want create a large volume by using software RAID, such as RAID-0, across hardware RAID LUNs.

  • You want to extend a volume, but the underlying hardware cannot dynamically increase the size of LUNs.

  • You want to extend a volume, but the hardware has reached its maximum LUN size.

Before converting hardware RAID disks to dynamic disks, review the following restrictions:

  • You cannot use dynamic disks on shared cluster storage. However, you can use the DiskPart command-line tool to extend basic volumes on shared cluster storage. For more information, see "Extend a basic volume" in Help and Support Center for Windows Server 2003.

  • If you create a software RAID-0 volume across multiple hardware arrays, you cannot later extend the RAID-0 volume to increase its size. If you anticipate needing to extend the volume, create a spanned volume instead.

Preparing to Upgrade Servers That Contain Multidisk Fault-Tolerant Volumes

Servers running Microsoft® Windows NT® Server version 4.0 can contain volume sets, mirror sets, stripe sets, and stripe sets with parity created by using the fault-tolerant driver Ftdisk.sys. To encourage administrators to begin using dynamic volumes, Windows 2000 offers limited support for Ftdisk volumes. Completing this transition, Windows Server 2003 does not support multidisk volumes. If you plan to upgrade a server that contains multidisk volumes, review the following issues:

  • If you are upgrading from Windows NT Server version 4.0 to Windows Server 2003, back up and then delete all multidisk volumes before upgrading. This is necessary because Windows Server 2003 cannot access these volumes. Be sure to verify that your backup was successful before deleting the volumes. After you finish upgrading to Windows Server 2003, create new dynamic volumes, and then restore the data from your backup.

  • If you are upgrading from Windows NT Server 4.0 to Windows Server 2003, and the paging file is located on a multidisk volume, you must use System in Control Panel to move the paging file to a primary partition or logical drive before beginning Setup. For more information about moving the paging file, see article 123747 in the Microsoft Knowledge Base (http://go.microsoft.com/fwlink/?linkid=99315).

  • If you are upgrading from Windows 2000 Server to Windows Server 2003, you must use the Disk Management snap-in to convert all basic disks that contain multidisk volumes to dynamic disks before beginning Setup. If you do not do this, Setup does not continue. For information about converting basic disks to dynamic disks, see "Change a basic disk into a dynamic disk" in Help and Support Center for Windows Server 2003.

For more information about multidisk volumes, see Storage Technologies Collection (http://go.microsoft.com/fwlink/?linkid=4681).

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