[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Levels | Technical Factors Differentiating RAID Levels ]

Like fault tolerance, performance is one of the main reasons that many people implement RAID arrays; and like fault tolerance, it is an important way that RAID levels differ greatly from one to the next. In fact, some RAID levels differ only in terms of performance.

Not only does performance vary between RAID levels, different types of performance vary between RAID levels. It is not meaningful to boil things down to just "performance"; rather, we must look at the issues of read and write performance; and positioning and transfer performance. RAID level "A" may be better than level "B" at one aspect of performance but much worse than "B" in another.

To make things more clear in explaining and contrasting the different RAID levels, I have combined these two two-element variables--read vs. write and positioning (random access) vs. transfer (sequential access)--into a "matrix" of four specific performance categories:

• Random Read Performance: How the RAID level performs on random access reads of files in the array. Typically, this is most important for transactional environments with smallish files, especially ones with a high ratio of reads to writes.
• Random Write Performance: How the RAID level performs when writing small files in random places on the array. Again, this is most relevant to transaction-processing environments, however, it is even more important to applications where a large number of writes are done, because write performance is much worse than read performance for many popular RAID levels.
• Sequential Read Performance: The performance of the RAID level when reading large files sequentially from the array. This is of greatest concern in applications where there are many more reads than writes, for example, a server containing many large graphics files.
• Sequential Write Performance: The RAID level's general performance when writing large files. This is sometimes less important than sequential read performance, but is critical for situations where large files are written often, such as video or audio editing.

Notes about performance assessments and comparisons (i.e., "very good", "fair", "poor", etc.) of RAID levels:

• All performance ratings are based on the assumption that hardware RAID is being used. Software RAID will often be substantially slower.
• Keep in mind that they are just rough generalizations; exact performance depends on disk type, controllers, stripe size, implementation details and a dozen other factors.
• Rankings are relative to other RAID levels. A "poor" rating means that RAID level does poorly on that type of transfer, even though it may still be faster than a single drive at that task.
• Write performance is generally worse than read performance, "scores" for write performance are generally rated against write performance of other RAID arrays.

Next: Multiple Array Considerations

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