What is RAID?
A technique was developed to provide speed, reliability, and increased storage capacity using multiple disks, rather than single disk solutions. RAID takes multiple hard drives and allows them to be used as one large hard drive with benefits depending on the scheme or level of RAID being used. The better the RAID implementation, the more expensive it is. There is no one best RAID implementation. Some implementations are better than others depending upon the actual application. It used to be that RAID was only available in expensive server systems. However, with the advent of inexpensive RAID controllers, it seems it has pretty much reached the mainstream market.
Mirroring
Mirroring involves having two copies of the same data on separate hard drives or drive arrays. So the data is effectively mirrored on another drive. The system writes data simultaneously to both hard drives. This is one of the two data redundancy methods used in RAID to protect from data loss.
RAID 0
This is the simplest level of RAID. 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 two hard drives are required, preferably identical, and the maximum depends upon the RAID controller.
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.
RAID 50
This level (also called RAID 0/5) is a combination of RAID levels 0 and 5. Multiple RAID 5 arrays are striped together using RAID 0. Parity is maintained separately for each RAID 5 group in the striped array. This level provides the same advantages of RAID 5 for small data transfers with the added performance of striping for disk read/write operations.
Exclusive OR
XOR is a logical operation used to generate a unique value for each pair of bits in a data word. XOR is used to calculate RAID-5 parity for data and also which can reconstruct the data using the previously calculated parity value.
Raid Implementation
Disk storage subsystems can be created through several different methods. These include a software based implementation, a bus-based model, and an external controller system. Each of these methods has its own unique advantages and disadvantages. The goals of the individual installation, including network size, cost, and performance needs, will determine which disk subsystem should be used.
Conclusion
RAID is a good solution for companies or individuals craving more transfer performance, redundancy, and storage capacity in their data storage systems. There are many levels of RAID, which range from very simple and cheap to extremely complex and expensive.
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