Different RAID Levels

Nov 18 22:00 2003 Ronald Merts Print This Article

... Types of RAID RAID Level 0RAID Level 0 or striping is ... for ... at the expense of fault ... Drives in a RAID 0 array ... data in such a way that it is striped ac

Different Types of RAID
RAID Level 0
RAID Level 0 or striping is optimized for performance at the expense of fault tolerance. Drives in a RAID 0 array organizes data in such a way that it is striped across the multiple drives. A RAID Level 0 array can contain any number of stripes. In RAID 0 if you have 2 x 60 gig drives the array size will be 120 gig.
The reason RAID 0 is a performance-enhancing configuration is that striping enables the array to access data from multiple drives at the same time. In other words,Guest Posting since the data is spread out across a number of drives in the array, it can be accessed faster because it's not bottled up on a single drive. This is especially beneficial for retrieving very large files, since they can be spread out effectively across multiple drives and accessed as if it were the size of any of the fragments it is organized into on the data stripes.
The downside to RAID Level 0 configurations is that it sacrifices fault tolerance, raising the risk of data loss because no room is made available to store redundant data. If one of the drives in the RAID 0 fails for any reason, there is no way of retrieving the lost data as can be done in other RAID implementations described below.
RAID Level 1
The RAID Level 1 is achieved through disk mirroring, and is done to ensure data reliability. RAID 1 also enhances read performance, but the improved performance and fault tolerance are at the expense of available capacity in the drives used. In RAID 1 If you have 2 x 60 gig drives the array size will be 60 gig.
In a RAID Level 1 configuration, the RAID management software instructs the subsystem's controller to store data redundantly across a number of the drives (mirrored set) in the array. In other words, the same data is copied and stored on different disks known as mirroring to ensure that, should a drive fail, the data is available somewhere else within the array. In fact, all but one of the drives in a mirrored set could fail and the data stored to the RAID 1 subsystem would remain intact. A RAID Level 1 configuration can consist of multiple mirrored sets, whereby each mirrored set can be a different capacity. Usually the drives making up a mirrored set are of the same capacity. If drives within a mirrored set are of different capacities, the capacity of a mirrored set within the RAID 1 subsystem is limited to the capacity of the smallest-capacity drive in the set.
The read performance gain can be realized if the redundant data is distributed evenly on all of the drives of a mirrored set within the subsystem. The number of read requests and total wait state times both drop significantly; inversely proportional to the number of hard drives in the RAID.
RAID Level 2
RAID Level 2 is rarely used in commercial applications, but is another means of ensuring data is protected in the event drives in the subsystem incur problems or otherwise fail. This level builds fault tolerance around Hamming error correction code (ECC), which is used as a means of maintaining data integrity. ECC tabulates the numerical values of data stored on specific blocks in the virtual drive using a special formula that yields what is known as a checksum. The check-sum is then appended to the end of the data block for verification of data integrity when needed. As data gets read back from the drive, ECC tabulations are again computed, and specific data block checksums are read and compared against the most recent tabulations. If the numbers match, the data is intact; if there is a discrepancy, the lost data can be recalculated using the first or earlier checksum as a reference point.
RAID Level 3
This RAID level is really an adaptation of RAID Level 0 that sacrifices some capacity, for the same number of drives, but achieves a high level of data integrity or fault tolerance. It takes advantage of RAID Level 0's data striping methods, except that data is striped across all but one of the drives in the array. This drive is used to store parity information that is used to maintain data integrity across all drives in the subsystem. The parity drive itself is divided up into stripes, and each parity drive stripe is used to store parity information for the corresponding data stripes dispersed throughout the array. This method achieves very high data transfer performance by reading from or writing to all of the drives in parallel or simultaneously but retains the means to reconstruct data if a given drive fails, maintaining data integrity for the system. RAID Level 3 is an excellent configuration for moving very large sequential files in a timely manner. The stripes of parity information stored on the dedicated drive are calculated using an "Exclusive OR" function, which is a logical function between the two series that carries most of the same attributes as the conventional OR function. The difference occurs when the two bits in the function are both non-zero: in Exclusive OR, the result of the function is zero, wherein with conventional OR it would be one.
RAID Level 4
RAID Level 4 is similar in concept to RAID Level 3, but emphasizes performance for different applications, e.g. Database TP versus large sequential files. Another difference between the two is that RAID Level 4 has a larger stripe depth, usually of two blocks, which allows the RAID management software to operate the disks much more independently than RAID Level 3. This essentially replaces the high data throughput capability of RAID Level 3 with faster data access in read-intensive applications.
A shortcoming of RAID level 4 is rooted in an inherent bottleneck on the parity drive. As data gets written to the array, the parity encoding scheme tends to be more tedious in write activities than with other RAID topologies. This more or less relegates RAID Level 4 to read-intensive applications with little need for similar write performance. As a consequence, like its Level 3, it doesn't see much common use in commercial applications.
RAID Level 5
This is the last of the most common RAID levels in use, and is probably the most frequently implemented. RAID Level 5 minimizes the write bottlenecks of RAID Level 4 by distributing parity stripes over a series of hard drives. In doing so it provides relief to the concentration of write activity on a single drive, which in turn enhances overall system performance. The way RAID Level 5 reduces parity write bottlenecks is relatively simple. Instead of allowing any one drive in the array to assume the risk of a bottleneck, all of the drives in the array assume write activity responsibilities. The distribution frees up the concentration on a single drive, improving overall subsystem throughput.
RAID Level 5's parity encoding scheme is the same as Levels 3 and 4; it maintains the system's ability to recover any lost data should a single drive fail. This can happen as long as no parity stripe on an individual drive stores the information of a data stripe on the same drive. In other words, the parity information for any data stripe must always be located on a drive other than the one on which the data resides.

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Ronald Merts
Ronald Merts

CIO/Sr. Vice President for Tornado Computers in Oklahoma City, OK. Located at http://www.tornadocomputers.com

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