Managing RAIDs and Recovering from Failures | Microsoft Windows Server 2003 Administrator[ap]s Pocket Consultant

Managing mirrored drives and striped sets is somewhat different from managing other drive volumes , especially when it comes to recovering from failure. The techniques you'll need to manage RAID arrays and to recover from failure are covered in this section.

Breaking a Mirrored Set

You might want to break a mirror for two reasons:

If one of the mirrored drives in a set fails, disk operations can continue. Here, when users read and write data, these operations use the remaining disk. Still, at some point you'll need to fix the mirror, and to do this, you must first break the mirror and then reestablish it.
If you no longer want to mirror your drives, you might also want to break a mirror. This allows you to use the disk space for other purposes.

Best Practices

Although breaking a mirror doesn't delete the data in the set, you should always back up the data before you perform this procedure. This ensures that if you have problems, you can recover your data.

In Disk Management, you can break a mirrored set by following these steps:

Right-click one of the volumes in the mirrored set and then choose Break Mirrored Volume.
Confirm that you want to break the mirror by clicking Yes. If the volume is currently in use, you'll see another warning dialog box. Confirm that it's okay to continue by clicking Yes.
Windows Server 2003 breaks the mirror, creating two independent volumes.

Resynchronizing and Repairing a Mirrored Set

Windows Server 2003 automatically synchronizes mirrored volumes on dynamic drives. However, data on mirrored drives can get out of sync. For example, if one of the drives goes offline, data is written only to the drive that's online.

You can resynchronize and repair mirrored sets on basic and dynamic disks, but you must rebuild the set using the same disk type. Follow these steps to resolve problems with a mirrored set:

You need to get both drives in the mirrored set online. The mirrored set's status should read Failed Redundancy. The corrective action you take depends on the failed volume's status.
If the status is Missing or Offline, make sure that the drive has power and is connected properly. Afterward, start Disk Management, right-click the failed volume, and select Reactivate Disk. The drive status should change to Regenerating and then to Healthy. If the volume doesn't return to the Healthy status, right-click the volume and then click Resynchronize Mirror.
If the status is Online (Errors), right-click the failed volume and select Reactive Disk. The drive status should change to Regenerating and then to Healthy. If the volume doesn't return to the Healthy status, right-click the volume and then click Resynchronize Mirror.
If one of the drives shows as Unreadable, you might need to rescan the drives on the system by selecting Rescan Disks from Disk Management's Action menu. If the drive status doesn't change, you might need to reboot the computer.
If one of the drives still won't come back online, right-click the failed volume and then select Remove Mirror. Next, right-click the remaining volume in the original mirror and then select Add Mirror. You'll now need to mirror the volume on an Unallocated area of free space. If you don't have free space, you'll need to create space by deleting other volumes or replacing the failed drive.

Repairing a Mirrored System Volume to Enable Boot

The failure of a mirrored drive might prevent your system from booting. Typically, this happens when you're mirroring the system or boot volume, or both, and the primary mirror drive has failed. In previous versions of the Windows operating system, you often had to go through several procedures to get the system back up and running. With Windows Server 2003, the failure of a primary mirror is much easier to resolve in most cases.

When you mirror a system volume, an entry that allows you to boot to the secondary mirror should be added to the system's Boot.ini file. The entry will look similar to the following:

 multi(0)disk(0)rdisk(2)partition(2)\WINNT="Boot Mirror D: - secondary plex"

With this entry in the Boot.ini file, resolving a primary mirror failure is much easier than before. To correct this problem, here are the steps you follow:

If a system fails to boot to the primary system volume, restart the system and select the Boot Mirror - Secondary Plex option for the operating system you want to start.
The system should start up normally.

Once you successfully boot the system to the secondary drive, you can schedule the maintenance necessary to rebuild the mirror if desired. You'll need to complete the following steps:

Shut down the system and replace the failed volume or add an additional hard disk drive. Afterward, restart the system.
Break the mirror set and then re-create the mirror on the drive you replaced , which is usually drive 0. Right-click the remaining volume that was part of the original mirror and then select Add Mirror. Next, follow the technique outlined in the section of this chapter entitled "Mirroring an Existing Volume."
If you want the primary mirror to be on the drive you added or replaced, use Disk Management to break the mirror again. Make sure that the primary drive in the original mirror set has the drive letter that was previously assigned to the complete mirror. If it doesn't, assign the appropriate drive letter.
Right-click the original system volume and then select Add Mirror. Now re-create the mirror.
Check Boot.ini and ensure that the original system volume is used during startup.

Removing a Mirrored Set

In Disk Management, you can remove one of the volumes from a mirrored set. When you do this, all data on the removed mirror is deleted and the space it used is marked as Unallocated.

To remove a mirror, complete the following steps:

In Disk Management, right-click one of the volumes in the mirrored set and then choose Remove Mirror. This displays the Remove Mirror dialog box.
In the Remove Mirror dialog box, select the disk from which to remove the mirror.
Confirm the action when prompted. All data on the removed mirror is deleted.

Caution

If the mirror contains a boot or system partition, you should check the settings in the Boot.ini file and then remove the mirror from the drive that isn't needed for system startup. For example, if rdisk(1) is used for startup and you have the choice of removing the mirror from Disk 1 or Disk 2, you'll usually want to remove the mirror from Disk 2. Removing the mirror from Disk 2 erases the redundant data.

Repairing a Striped Set Without Parity

A striped set without parity doesn't have fault tolerance. If a drive that's part of a striped set fails, the entire striped set is unusable. Before you try to restore the striped set, you should repair or replace the failed drive. Afterward, you need to recreate the striped set and then recover the data contained on the striped set from backup.

Regenerating a Striped Set with Parity

With RAID 5 you can recover the striped set if a single drive fails. You'll know that a striped set with parity drive has failed because the set's status changes to Failed Redundancy and the individual volume's status changes to Missing, Offline, or Online (Errors).

You can repair RAID 5 on basic and dynamic disks, but you must rebuild the set using the same disk type. Follow these steps to resolve problems with the RAID-5 set:

You need to get all drives in the RAID-5 set online. The set's status should read Failed Redundancy. The corrective action you take depends on the failed volume's status.

Best Practices

If possible, you should back up the data before you perform this procedure. This ensures that if you have problems, you can recover your data.
If the status is Missing or Offline, make sure that the drive has power and is connected properly. Afterward, start Disk Management, right-click the failed volume, and select Reactivate Disk. The drive's status should change to Regenerating and then to Healthy. If the drive's status doesn't return to Healthy, right-click the volume and select Regenerate Parity.
If the status is Online (Errors), right-click the failed volume and select Reactivate Disk. The drive's status should change to Regenerating and then to Healthy. If the drive's status doesn't return to Healthy, right-click the volume and select Regenerate Parity.
If one of the drives shows as unreadable, you might need to rescan the drives on the system by selecting Rescan Disks from Disk Management's Action menu. If the drive status doesn't change, you might need to reboot the computer.
If one of the drives still won't come back online, you need to repair the failed region of the RAID-5 set. Right-click the failed volume and then select Remove Volume. You now need to select an unallocated space on a separate dynamic disk for the RAID-5 set. This space must be at least as large as the region to repair, and it can't be on a drive that's already being used by the RAID-5 set. If you don't have enough space, the Repair Volume command is unavailable and you'll need to free space by deleting other volumes or replacing the failed drive.