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Running benchmarks in a controlled environment is a useful method for comparing performance, such as the Ziff-Davis Winbench® 97 benchmark results contained in the AAA®-131CA PCI Array Card Series report. This report concludes that Adaptec's hardware-based RAID solutions demonstrate a consistent performance advantage over NT software-based RAID. Certain applications such as NASTRAN, Adobe AfterEffects, Adobe Photoshop and AutoCAD may see a significant performance improvement due to card-based caching used on AAA-131CA workstation cards because of more efficient cache flush operations, reduced disk drive head thrashing, fewer cache misses, and more writes at memory rather than disk speeds.

But just as your car mileage may vary from EPA mileage ratings, performance on your RAID storage will vary based on your system configuration and application environment. Whether the performance differences are enough to warrant selecting a hardware-based solution is the tricky part. However, since most applications can be characterized as being CPU-bound, I/O-bound, or a mixture of both, an empirical discussion of software- and hardware-based RAID solutions may be helpful in determining which solution is best for you.

The argument in favor of hardware-based RAID in CPU-bound applications is straight forward one. Offloading RAID 5 parity calculations and RAID 1 secondary writes to a separate hardware-based RAID co-processor reduces CPU interrupts, freeing the main CPU to perform other compute-intensive functions. I/O traffic on the main PCI bus is reduced, so that other activities such as network traffic can be processed more efficiently.

The performance advantages of hardware-based RAID is especially pronounced when RAID 5 data sets are operating in degraded mode (i.e., a drive in the array has failed), because both read and write requests require parity calculations, significantly increasing CPU interrupts and I/O traffic.

In I/O-bound applications, the differences between software- and hardware-based RAID are less apparent. Clearly, if disk drives are the bottleneck, whether the parity calculations are performed in the main CPU or RAID co-processor will make little difference in overall system performance. However, there are some situations where hardware-based RAID may be advantageous.

You could see a significant improvement in mirrored drive (RAID 1) performance if you implement striping and mirroring (RAID 0/1), not available on Windows NT or Netware software-based RAID implementations. With RAID 0/1, not only could your application experience improved read and write times due to simultaneous multiple drive accesses, but also more consistent and predictable performance due to the load balancing effect of RAID 0.

If your application is already I/O-bound, a failed drive in a RAID 5 data set can have a paralyzing effect on system performance. Hardware-based RAID solutions that support automatic failed drive detection with hot spare replacement can significantly reduce the amount of time your application is running in degraded mode, because the application does not have to wait until you physically replace the failed drive. Hardware-based RAID solutions that allow you to set the priority (low/medium/high) for array reconstruction, gives you control over the tradeoffs you are willing to make between overall system performance and availability.

Hardware-based RAID solutions can improve system boot time and operating system performance by striping the operating system files, a feature not supported on NT's software-based RAID implementations.

Since we have just identified performance advantages of hardware-based RAID for both CPU-bound and I/O-bound applications, mixed I/O environments are likely to also experience improved performance with a hardware-based RAID solution.