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Grouping Similar Disk Activities
Configuring one large RAID 5 array to handle all your disk I/O needs might appear to be the easiest solution, but this approach is not always a wise decision. You can dramatically improve performance by matching the performance characteristics of each RAID level with your disk workload patterns. For example, a Microsoft Exchange Server environment contains both sequentially write-intensive log files and random information-store data files. Instead of using one RAID 5 array for both activities, you'll achieve the greatest performance by placing the write-intensive log files on their own RAID 1 array and leaving the random information-store data files on the RAID 5 array. This approach provides better performance because you're moving the write-intensive workload away from the RAID 5 array, which exhibits slower write performance than a RAID 1 array exhibits. Configuring the RAID levels to match your workload patterns improves the response times of the disk subsystem and, ultimately, the NT system.
If you use a stress-testing tool to measure your server's performance, you can quantify the overall server performance benefit of using multiple RAID levels or adding extra standalone disks. If you don't have a stress-testing tool available, you can use two counters (Avg. Disk sec/Write and Avg. Disk sec/Read) under the LogicalDisk object in NT's Performance Monitor to help you determine whether using multiple RAID levels or standalone disks will increase performance. The Avg. Disk sec/Write counter measures the average time in seconds to write data to the disk, and the Avg. Disk sec/Read counter measures the average time in seconds to read data from the disk. Look at these two counters before and after you change your disk subsystem configuration. If the workload on your server is roughly the same before and after you make changes to your disk subsystem, you will see significant improvements in these two metrics after you implement multiple RAID levels or add standalone disks. Remember, these values will always be zero if you haven't run diskperf -ye from the NT command prompt and rebooted your NT system to activate NT's collection of disk metrics. Numerous other techniques and Performance Monitor counters can help you measure increased application performance after you have made your changes to your disk subsystem. For more detailed and advanced information, check out Tuning and Sizing of Windows 2000 for Maximum Peformance.