Levels of RAID
In the previous sections I dealt with the basic concepts behind RAID. All those concepts are used to make up the various RAID levels out there now. I'm covering the standard RAID levels here. Some companies have developed their own proprietary RAID levels which are not covered here. Also, others might have different interpretations of the levels of RAID. The explanation I present here are what I consider to be these levels. These merely the single RAID levels. These levels can be combined in many different ways to provide more functionality. I'll get into multiple or nested RAID levels in the next section.
RAID 0
This is the simplest level of RAID, and it just involves striping. Data redundancy is not even present in this level, so it is not recommended for applications where data is critical. This level offers the highest level of performance out of any single RAID level. It also offers the lowest cost since no extra storage is involved. At least 2 hard drives are required, preferably identical, and the maximum depends on the RAID controller. None of the space is wasted as long as the hard drives used are identical. This level has become popular with the mainstream market for it's relatively low cost and high performance gain. This level is good for most people that don't need any data redundancy. There are many SCSI and IDE/ATA implementations available. Finally, it's important to note that if any of the hard drives in the array fails, you lose everything.
RAID 1
This level is usually implemented as mirroring. Two identical copies of data are stored on two drives. When one drive fails, the other drive still has the data to keep the system going. Rebuilding a lost drive is very simple since you still have the second copy. This adds data redundancy to the system and provides some safety from failures. Some implementations add an extra RAID controller to increase the fault tolerance even more. It is ideal for applications that use critical data. Even though the performance benefits are not great, some might just be concerned with preserving their data. The relative simplicity and low cost of implementing this level has increased its popularity in mainstream RAID controllers. Most RAID controllers nowadays implement some form of RAID 1.
RAID 2
This level uses bit level striping with Hamming code ECC. The technique used here is somewhat similar to striping with parity but not really. The data is split at the bit level and spread over a number of data and ECC disks. When data is written to the array, the Hamming codes are calculated and written to the ECC disks. When the data is read from the array, Hamming codes are used to check whether errors have occurred since the data was written to the array. Single bit errors can be detected and corrected immediately. This is the only level that really deviates from the RAID concepts talked about earlier. The complicated and expensive RAID controller hardware needed and the minimum number of hard drives required, is the reason this level is not used today.
RAID 3
This level uses byte level striping with dedicated parity. In other words, data is striped across the array at the byte level with one dedicated parity drive holding the redundancy information. The idea behind this level is that striping the data increasing performance and using dedicated parity takes care of redundancy. 3 hard drives are required. 2 for striping, and 1 as the dedicated parity drive. Although the performance is good, the added parity does slow down writes. The parity information has to be written to the parity drive whenever a write occurs. This increased computation calls for a hardware controller, so software implementations are not practical. RAID 3 is good for applications that deal with large files since the stripe size is small.
RAID 4
This level is very similar to RAID 3. The only difference is that it uses block level striping instead of byte level striping. The advantage in that is that you can change the stripe size to suit application needs. This level is often seen as a mix between RAID 3 and RAID 5, having the dedicated parity of RAID 3 and the block level striping of RAID 5. Again, you'll probably need a hardware RAID controller for this level. Also, the dedicated parity drive continues to slow down performance in this level as well.
RAID 5
RAID 5 uses block level striping and distributed parity. This level tries to remove the bottleneck of the dedicated parity drive. With the use of a distributed parity algorithm, this level writes the data and parity data across all the drives. Basically, the blocks of data are used to create the parity blocks which are then stored across the array. This removes the bottleneck of writing to just one parity drive. However, the parity information still has to be calculated and written whenever a write occurs, so the slowdown involved with that still applies. The fault tolerance is maintained by separating the parity information for a block from the actual data block. This way when one drive goes, all the data on that drive can be rebuilt from the data on the other drives. Recovery is more complicated than usual because of the distributed nature of the parity. Just as in RAID 4, the stripe size can be changed to suit the needs of the application. Also, using a hardware controller is probably the more practical solution. RAID 5 is one of the most popular RAID levels being used today. Many see it as the best combination of performance, redundancy, and storage efficiency.
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