Planning for Fault Tolerant Disks

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Your cluster design needs to implement the use of fault-tolerant disks. Although we won't delve deeply into the use of fault-tolerant disks, where and how you should implement them when the need occurs will be highlighted. As of this section, you need to know where fault-tolerant disks come up in the overall design. When you plan for fault-tolerant disks, you should consider RAID. RAID support makes sure the data contained on your clustered disk sets is highly available. Hardware RAID, which can be implemented in a shared device among the cluster members, can almost guarantee you won't lose data or make sure it's recoverable if a disaster occurs. You should factor into your initial design that you can't use software fault-tolerant-based disk sets for cluster storage. Also, always consult the Microsoft Hardware Compatibility List (HCL) for any hardware purchasing you plan to do, especially with extravagant and expensive hardware solutions such as RAID and clustering solutions. If you're going to implement a RAID solution into your High Availability design (wise choice), then you need to consider which version of RAID you want to implement.

Raid Version

Fault Tolerant?

Raid 0


Raid 1


Raid 5


Raid 0+1


When you configure RAID, you'll want to design at least one of the most popular and functional versions of RAID into your infrastructure. RAID 0 is used only as a speed enhancement, enabling multiple drives to be written to and read from simultaneously.

RAID 0 is disk striping without parity. Although it accounts for faster disk reads and writes, no fault tolerance is involved whatsoever in RAID 0. If a disk failure occurs, you can't rebuild the rest of the data by inserting a new disk into the set. Raid 1 is the beginning of fault tolerance within RAID, but it's slower, depending on which version of RAID 1 you implement. RAID 1 with mirroring is achieved by using two disks within a system on the same motherboard controller. When data is written to one disk, it's then written to the second disk achieving fault tolerance.

When one disk fails, the other has a working version of the data ready to go. With mirroring, you have a single point of failure, which is removed from the equation when you implement RAID 1 disk duplexing. This is the same as mirroring, except you're now working from two disk controllers on the motherboard instead of one. RAID 5 is the fastest and most common RAID version used today that also offers fault tolerance. Disk striping with parity (RAID 0 does not have this) offers fast reads and writes, while maintaining a separate disk to store parity information. This will be essential to re-create the disk if a failure occurs. Raid 0+1 or (RAID 10) is the combination of RAID levels 0 and 1. For design purposes, you need to implement something highly fault-tolerant if you want to maintain a highly available posture to your clients accessing resources. Cost is the only factor from this point. RAID 5 and RAID 10 are the best options, but they cost the most. Examples of RAID 0,1, and 5 can be seen in Figure 1-17.

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