Implementing RAID on Windows Server 2003

Windows Server 2003 supports disk mirroring, disk striping, and disk striping with parity. Implementing these RAID techniques is discussed in the sections that follow.

Caution

Some operating systems, such as MS-DOS, don't support RAID. If you dual boot your system to one of these noncompliant operating systems, your RAID-configured drives will be unusable.

Implementing RAID 0: Disk Striping

RAID level 0 is disk striping. With disk striping, two or more volumes ”each on a separate drive ”are configured as a striped set. Data written to the striped set is broken into blocks that are called stripes . These stripes are written sequentially to all drives in the striped set. You can place volumes for a striped set on up to 32 drives, but in most circumstances sets with two to five volumes offer the best performance improvements. Beyond this, the performance improvement decreases significantly.

The major advantage of disk striping is speed. Data can be accessed on multiple disks using multiple drive heads, which improves performance considerably. However, this performance boost comes with a price tag. As with volume sets, if any hard disk drive in the striped set fails, the striped set can no longer be used, which means that essentially all data in the striped set is lost. You'll need to recreate the striped set and restore the data from backups . Data backup and recovery is discussed in Chapter 15 , "Data Backup and Recovery."

Caution

The boot and system volumes shouldn't be part of a striped set. Don't use disk striping with these volumes.

When you create striped sets, you'll want to use volumes that are approximately the same size. Disk Management bases the overall size of the striped set on the smallest volume size. Specifically, the maximum size of the striped set is a multiple of the smallest volume size. For example, if the smallest volume is 50 MB, the maximum size for the striped site is 150 MB.

To maximize performance of the striped set, you can do several things:

Use disks that are on separate disk controllers. This allows the system to simultaneously access the drives.
Don't use the disks containing the striped set for other purposes. This allows the disk to dedicate its time to the striped set.

You can create a striped set by doing the following:

In the Disk Management Graphical View, right-click an area marked Unallocated on a dynamic disk and then choose New Volume. This starts the New Volume Wizard. Read the welcome page, and then click Next.
Select Striped as the volume type. Create the volume as described previously in this chapter under "Creating Volumes and Volume Sets." The key difference is that you need at least two dynamic disks to create a striped volume.
Once you create a striped volume, you can use the volume just like any other volume. You can't expand a striped set once it's created. Because of this, you should carefully consider the setup before you implement it.

Implementing RAID 1: Disk Mirroring

RAID level 1 is disk mirroring. With disk mirroring, you use identically sized volumes on two different drives to create a redundant data set. Here, the drives are written with identical sets of information and, if one of the drives fails, you can still obtain the data from the other drive.

Disk mirroring offers about the same fault tolerance as disk striping with parity. Because mirrored disks don't need to write parity information, they can offer better write performance in most circumstances. However, disk striping with parity usually offers better read performance because read operations are spread out over multiple drives.

The major drawback to disk mirroring is that it effectively cuts the amount of storage space in half. For example, to mirror a 5-GB drive, you need another 5-GB drive. That means you use 10 GB of space to store 5 GB of information.

Tip

If possible, it's a good idea to mirror boot and system volumes. Mirroring these volumes ensures that you'll be able to boot the server in case of a single drive failure.

As with disk striping, you'll often want the mirrored disks to be on separate disk controllers. This provides increased protection against failure of the disk controller. If one of the disk controllers fails, the disk on other controller is still available. Technically, when you use two separate disk controllers to duplicate data, you're using a technique known as disk duplexing . Figure 12-5 shows the difference between the two techniques. Where disk mirroring typically uses a single drive controller, disk duplexing uses two drive controllers.

Figure 12-5. Although disk mirroring typically uses a single drive controller to create a redundant data set, disk duplexing uses two drive controllers. Other than this, the two techniques are essentially the same.

graphics/f12ap05.jpg

If one of the mirrored drives in a set fails, disk operations can continue. Here, when users read and write data, the data is written to the remaining disk. You'll need to break the mirror before you can fix it. To learn how, see the section of this chapter entitled "Managing RAIDs and Recovering from Failures."

Creating a Mirror Set in Disk Management

You create a mirror set by completing the following steps:

In the Disk Management Graphical View, right-click an area marked Unallocated on a dynamic disk and then choose New Volume. This starts the New Volume Wizard. Read the welcome page, and then click Next.
Select Mirrored as the volume type. Create the volume as described previously under "Creating Volumes and Volume Sets." The key difference is that you must create two identically sized volumes and these volumes must be on separate dynamic drives. You won't be able to continue past the Selected Disks window until you've selected the two disks that you want to work with.
As with other RAID techniques, mirroring is transparent to users. Users see the mirrored set as a single drive that they can access and use like any other drive.

More Info

The status of a normal mirror is Healthy. During the creation of a mirror, you'll see a status of Resynching. This tells you that Disk Management is creating the mirror.

Mirroring an Existing Volume

Rather than creating a new mirrored volume, you can use an existing volume to create a mirrored set. To do this, the volume you want to mirror must be a simple volume and you must have an area of unallocated space on a second dynamic drive of equal or larger space than the existing volume.

In Disk Management, you mirror an existing volume by completing the following steps:

Right-click the simple volume you want to mirror and then select Add Mirror. This displays the Add Mirror dialog box.
Use the Disks list box to select a location for the mirror and then click Add Mirror. Windows Server 2003 begins the mirror creation process and you'll see a status of Resynching on both volumes, as shown in Figure 12-6.

Figure 12-6. When the mirror is being created, you see a status of Resynching. The disk on which the mirrored volume is being created also has a warning icon.

Implementing RAID 5: Disk Striping with Parity

RAID level 5 is disk striping with parity. With this technique you need a minimum of three hard disk drives to set up fault tolerance. Disk Management sizes the volumes on these drives identically.

RAID 5 is essentially an enhanced version of RAID 1 ”with the key addition of fault tolerance. Fault tolerance ensures that the failure of a single drive won't bring down the entire drive set. Instead, the set continues to function with disk operations directed at the remaining volumes in the set.

To allow for fault tolerance, RAID 5 writes parity checksums with the blocks of data. If any of the drives in the striped set fails, the parity information can be used to recover the data. (This process, called regenerating the striped set, is covered in the section of this chapter entitled "Managing RAIDs and Recovering from Failures.") If two disks fail, however, the parity information isn't sufficient to recover the data and you'll need to rebuild the striped set from backup.

Creating a Striped Set with Parity in Disk Management

In Disk Management, you can create a striped set with parity by completing the following steps:

In the Disk Management Graphical View, right-click an area marked Unallocated on a dynamic disk and then choose New Volume. This starts the New Volume Wizard. Read the welcome page, and then click Next.
Select RAID-5 as the volume type. Create the volume as described previously under "Creating Volumes and Volume Sets." The key difference is that you must select free space on three separate dynamic drives.
Once you create a striped set, users can use the set just like they would a normal drive. Keep in mind that you can't expand a striped set once it's created. Because of this, you should carefully consider the setup before you implement it.