Windows 7 / Getting Started

---

Configuring RAID 5: Disk Striping with Parity

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 always 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, and 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, 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.

Okay, 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 a 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 had three 60-GB drives using RAID 5, you'd use 180 GB of space to store about 120 GB 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, 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. Click Next. The key difference is that you must select free space on three or more separate dynamic drives.

When you 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.