Each RAID level defines a different way to spread data across multiple drives. This usually requires a compromise between cost and speed. Understanding the differences between these levels is important, because each level is optimized for a different use.
RAID Level 0
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RAID Level 0 provides no redundancy. RAID Level 0 splits or stripes the data across drives, resulting in higher data throughput. Since no redundant information is stored, performance is very good, but the failure of any disk in the array results in complete data loss. Level 0 is only used to increase disk performance.
RAID Level 1
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RAID Level 1 is usually referred to as mirroring. A Level 1 array provides redundancy by duplicating all the data from one drive on a second drive so that if either drive fails, no data is lost. This is a good entry-level redundant system. Most newer motherboards have this option built-in today using the Promise Technology IDE RAID chip. The downside to this is that the cost per megabyte of disk storage is twice that of a single drive as two drives are needed to store the same data.
RAID Level 3
RAID Level 3 stripes data at a byte level across several drives, with parity stored on one drive. Byte-level striping requires hardware support for good performance. If one disk fails it is possible to rebuild the complete data set so that no data is lost. If more than one drive fails all the stored data will be lost. The cost per megabyte is lower than RAID level 1 but the performance is lower than RAID level 5.
RAID Level 5
RAID Level 5 stripes data at a block level across several drives and distributes parity among the drives. No single disk is devoted to parity. This can speed small writes in multiprocessing systems. Because parity data is distributed on each drive, read performance tends to be lower than other RAID types.
The actual amount of available storage is about 75% to 80% of the total storage in the disk array. The storage penalty for redundancy is only about 20% of the total storage in the array. If one disk fails it is possible to rebuild the complete data set so that no data is lost. If more than one drive fails all the stored data will be lost. This give a fairly low cost per megabyte while still retaining redundancy.
RAID 0+1 or RAID 10
RAID 0+1 or RAID 10 is a combination of RAID Levels that utilizes multiple RAID1 (mirrored) sets into a single array. Data is striped across all mirrored sets. As a comparison to RAID 5 where lower cost and fault tolerance is important, RAID 0+1 utilizes several drives to stripe data (increased performance) and then makes a copy of the striped drives to provide redundancy. Any disk can fail and no data is lost as long as the mirror of that disk is still operational. The mirrored disks eliminate the overhead and delay of parity. This level array offers high data transfer advantages of striped arrays and increased data accessibility (reads). System performance during a drive rebuild is also better than that of parity based arrays, since data does not need to be regenerated from parity information, but is copied from the other mirrored drive.
RAID 0+5 or RAID 50
RAID 0+5 or RAID 50 is a combination of RAID levels that utilizes multiple RAID 5 sets striped in a single array. In a RAID 0+5 array, a single hard drive failure can occur in each of the RAID 5 sides without any loss of data on the entire array. If, however more than one disk is lost in any of the RAID 5 arrays all the data in the array is lost. As the number of hard drives increase in an array, so does the possibility of a single hard drive failure. Although there is an increased write performance in RAID 0+5, once a hard drive fails and reconstruction takes place, there is a noticeable decrease in performance, data/program access will be slower, and transfer speeds on the array will be effected.
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