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Combining Levels of RAID
The single RAID levels don't address every application requirement that exist. So, to get more functionality, someone thought of the idea of combining RAID levels. What if you can combine two levels and get the advantages of both? Well that was the motivation behind creating these new levels. The main benefit of using multiple RAID levels is the increased performance. Usually combining RAID levels means using a hardware RAID controller. The increased level of complexity of these levels means that software solutions are not practical. RAID 0 has the best performance out of the single levels and it is the one most commonly being combined. Not all combinations of RAID levels exist. The most common combinations are RAID 0+1 and 1+0. The difference between 0+1 and 1+0 might seem subtle, and sometimes companies may use the terms interchangeably. However, the difference lies in the amount of fault tolerance. Both these levels require at least 4 hard drives to implement. Let's look at RAID 0+1 first.
This combination uses RAID 0 for it's high performance and RAID 1 for it's high fault tolerance. I actually mentioned this level when I talked about adding striping to mirroring. Let's say you have 8 hard drives. You can split them into 2 arrays of 4 drives each, and apply RAID 0 to each array. Now you have 2 striped arrays. Then you would apply RAID 1 to the 2 striped arrays and have one array mirrored on the other. If a hard drive in one striped array fails, the entire array is lost. The other striped array is left, but contains no fault tolerance if any of the drives in it fail.
RAID 1+0 applies RAID 1 first then RAID 0 to the drives. To apply RAID 1, you split the 8 drives into 4 sets of 2 drives each. Now each set is mirrored and has duplicate information. To apply RAID 0, you then stripe across the 4 sets. In essence, you have a striped array across a number of mirrored sets. This combination has better fault tolerance than RAID 0+1. As long as one drive in a mirrored set is active, the array can still function. So theoretically you can have up to half the drives fail before you lose everything, as opposed to only two drives in RAID 0+1.
The popularity of RAID 0+1 and 1+0 stems from the fact that it's relatively simple to implement while providing high performance and good data redundancy. With the increased reduction of hard drive prices, the 4 hard drive minimum isn't unreasonable to the mainstream anymore. However, you still have the 50% waste in storage space whenever you are dealing with mirroring. Enterprise applications and servers are often willing to sacrifice storage for increased performance and fault tolerance. Some other combinations of RAID levels that are used include, RAID 0+3, 3+0, 0+5, 5+0, 1+5, and 5+1. These levels are often complicated to implement and require expensive hardware. Not all of the combinations I mentioned above are used
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