Managing Windows Server 2012 Storage and File Systems : Storage Management (part 14) - Managing volumes on dynamic disks - Configuring RAID 5

RAID 5, disk striping with parity, offers fault tolerance with less overhead and better read performance than disk mirroring. To configure RAID 5, you use three or more volumes, each on a separate drive, as a striped set, similar to RAID 0. Unlike RAID 0, however, RAID 5 adds parity error checking to ensure that the failure of a single drive won’t bring down the entire drive set. In the event of a single drive failure, the set continues to function with disk operations directed at the remaining disks in the set. The parity information can also be used to recover the data using a process called regeneration.

RAID 5 works like this: Each time the operating system writes to a RAID-5 volume, the data is written across all the disks in the set. Parity information for the data, used for error checking and correction, is written to disk as well, but it’s always written on a separate disk from the one used to write the data. For example, if you are using a three-volume RAID-5 set and save a file, the individual data bytes of the file are written to each of the disks in the set. Parity information is written as well, but not to the same disk as one of the individual data bytes. Thus, a disk in the set could have a chunk of the data or the corresponding parity information, but not both. This, in turn, means that the loss of one disk from the set doesn’t cause the entire set to fail.

Like any type of RAID, RAID 5 has its drawbacks as well. First, if multiple drives in the set fail, the entire set will fail and you won’t be able to regenerate the set from the parity information. Why? If multiple drives fail, there won’t be enough parity information to use to recover the set. Second, having to generate and write parity information every time data is written to disk slows down the write process (and, in the case of software RAID, reduces processing power). To compensate for the performance hit, hardware RAID controllers have their own processors that handle the necessary processing—and this is why hardware RAID is preferred over software RAID.

OK, so RAID 5 gives you fault tolerance at some cost to performance. It does, however, have less overhead than RAID 1. By using RAID 1, you have 50 percent overhead, which effectively cuts the amount of storage space in half. By using RAID 5, the overhead depends on the number of disks in the RAID set. With three disks, the overhead is about one-third. If you have three 750-GB drives using RAID 5, you use 2250 GBs of space to store about 1500 GBs of information. If you have additional disks, the overhead is reduced incrementally, but not significantly.

To create a RAID-5 set, start Disk Management. In Graphical View, press and hold or right-click an area marked Unallocated on a dynamic disk and then choose New RAID-5 Volume. This starts the New RAID-5 Volume Wizard. Tap or click Next. Create the volume . The key difference is that you must select free space on three or more separate dynamic drives.

When you tap or click Finish, you’ll return to the main Disk Management window and Disk Management will create the RAID-5 set. During the creation of the mirror, you’ll see a status of Resynching. This tells you that Disk Management is creating the RAID-5 set. When this process finishes, you’ll have three or more identical volumes, all of which will show the same drive letter in Disk Management. Users, however, will see the RAID-5 set as a single volume. The volume status should be listed as Healthy. This is the normal status for volumes. If the status changes, you might need to repair or regenerate the RAID-5 set .

Windows Server 2012 provides two ways to stop mirroring. You can break a mirrored set, creating two separate but identical volumes. Or you can remove a mirror, which deletes all the data on the removed mirror.

To break a mirrored set, follow these steps:

To remove a mirror, follow these steps:

Occasionally, data on mirrored volumes can get out of sync. Typically, this happens if one of the drives in the set goes offline or experiences temporary I/O problems and, as a result, data can be written only to the drive that’s online. To reestablish mirroring, you must get both drives online and then resynchronize the mirror, but you must rebuild the set using a disk with the same partition style—either MBR or GPT. The corrective action you take depends on the drive status.

The Missing or Offline status usually happens if drives have been disconnected or powered off. If the drives are part of an external storage device, check the storage device to ensure that it is connected properly and has power. Reconnecting the storage device or turning on the power should make it so that the drives can be accessed. You then must start Disk Management and rescan the missing drive by selecting Rescan Disks on the Action menu. When Disk Management finishes, press and hold or right-click the drive and choose Reactivate Volume. The drive status should change to Regenerating and then to Healthy. If the volume doesn’t return to the Healthy status, press and hold or right-click the volume and then choose Resynchronize Mirror.

A status of Failed, Online (Errors), or Unreadable indicates I/O problems with the drive. As before, try rescanning the drive, and then try to reactivate the drive. The drive status should change to Regenerating and then to Healthy. If the volume doesn’t return to the Healthy status, press and hold or right-click the volume and then choose Resynchronize Mirror.

If these actions don’t work, you must remove the failed mirror, replace the bad drive, and then rebuild the mirror. To do this, follow these steps:

When you mirror a system volume, an entry that allows you to boot to the secondary mirror is added to the system’s boot configuration data. So, 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. After you successfully boot the system to the secondary drive, you can schedule the maintenance necessary to rebuild the mirror if desired.