What is RAID?
RAID is an acronym for Redundant Array of Inexpensive (or Independent) Disks. A RAID array is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other.
What are the different RAID levels?
A research group at UC-Berkeley coined the term "RAID", defining six RAID levels. Each level is a different way to spread data across multiple drives--a compromise between cost and speed. Understanding these levels is important, because each level is optimized for a different use.
RAID Level 0 is not redundant, hence does not truly fit the "RAID" acronym. In Level 0, data is split 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 all data loss. This level is commonly referred to as striping.
RAID Level 1 is commonly referred to as mirroring with 2 hard drives. It provides redundancy by duplicating all data from one drive on another drive. The performance of a Level 1 array is slightly better than a single drive, but if either drive fails, no data is lost. This is a good entry-level redundant system, since only two drives are required. However, since one drive is used to store a duplicate of the data, the cost per megabyte is high.
RAID Level 2, which uses Hamming error correction codes, is intended for use with drives which do not have built-in error detection. All SCSI drives support built-in error detection, so this level is of little use when using SCSI drives.
RAID Level 3 stripes data at a byte level across several drives, with parity stored on one drive. It is otherwise similar to level 4. Byte-level striping requires hardware support for efficient use.
RAID Level 4 stripes data at a block level across several drives, with parity stored on one drive. The parity information allows recovery from the failure of any single drive. The performance of a level 4 array is very good for reads (the same as level 0). Writes, however, require that parity data be updated each time. This slows small random writes, in particular, though large writes or sequential writes are fairly fast. Because only one drive in the array stores redundant data, the cost per megabyte of a level 4 array can be fairly low.
RAID Level 5 is commonly referred to as striping with distributed parity. RAID Level 5 is similar to level 4, but distributes parity among the drives. No single disk is devoted to parity. This can speed small writes in multiprocessing systems. Because parity data must be distributed on each drive during reads, the performance for reads tends to be considerably lower than a level 4 array. The cost per megabyte is the same as for level 4.
RAID 0/1 or 10 is a dual level array 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, in order to provide better performance. Each drive in the array is duplicated (mirrored). This eliminates 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 copied from the other mirrored drive.
RAID 0/5 or 50 is a dual level array that utilizes multiple RAID5 sets into a single array. In RAID 0/5 array, a single hard drive failure can occur in each of the RAID5 without any loss of data on the entire array. Keep in mind, as the number of hard drives increase in an array, so too, does the increased 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.