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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Redundant Arrays of Inexpensive Disks (RAID) | RAID Concepts and Issues | RAID Performance Issues ]
Read and Write Performance
Hard disks perform two distinct functions: writing data, and then reading it back. In most ways, the electronic and mechanical processes involved in these two operations are very similar. However, even within a single hard disk, read and write performance are often different in small but important ways. This is discussed in more detail here. When it comes to RAID, the differences between read and write performance are magnified. Because of the different ways that disks can be arranged in arrays, and the different ways data can be stored, in some cases there can be large discrepancies in how "method A" compares to "method B" for read performance, as opposed to write performance.
The fundamental difference between reading and writing under RAID is this: when you write data in a redundant environment, you must access every place where that data is stored; when you read the data back, you only need to read the minimum amount of data necessary to retrieve the actual data--the redundant information does not need to be accessed on a read. OK, this isn't as complicated as I probably just made it sound. :^) Let's see how various storage techniques used in RAID differ in this regard:
Note: As if the
performance hit for writes under striping with parity weren't bad enough, there is even
one more piece of overhead! The controller has to make sure that when it changes data and
its associated parity, all the changes happen simultaneously; if the process were
interrupted in the middle, say, after the data were changed and not the parity, the
integrity of the array would be compromised. To prevent this, a special process must be
used, sometimes called a two-phase commit. This is similar to the techniques used
in database operations, for example, to make sure that when you transfer money from your
checking account to your savings account, it doesn't get subtracted from one without being
certain that it was added to the other (or vice-versa). More overhead, more performance
The bottom line that results from the difference between read and write performance is that many RAID levels, especially ones involving striping with parity, provide far better net performance improvement based on the ratio of reads to writes in the intended application. Some applications have a relatively low number of writes as a percentage of total accesses; for example, a web server. For these applications, the very popular RAID 5 solution may be an ideal choice. Other applications have a much higher percentage of writes; for example, an interactive database or development environment. These applications may be better off with a RAID 01 or 10 solution, even if it does cost a bit more to set up.
Note: Some controllers
employ write caching to improve performance during writes; see here for more on this advanced feature.