Tips for keeping your system in tip-top shape
Performance! Everyone wants Windows NT servers and workstations to run as fast as possible. Keeping your disk subsystem running at its best is an important step in improving the overall performance of your NT solution. In my article, "Sizing Your NT RAID Array," August 1998, I showed you how to determine when your NT disk subsystem becomes a bottleneck and how to size additional disk capacity for your NT system. But how do you get the most from an additional standalone hard disk or RAID array investment?
In this article, I'll show you how to maximize the performance of your extra disk capacity when implementing NT disk subsystems and RAID arrays, regardless of any vendor-specific hardware tweaks. You can use NT's built-in tools and a freeware tool to quickly optimize your disk subsystem (hardware- or software-based RAID). To accomplish this task, you must understand the performance characteristics of the standalone disk or RAID technology you are working with and the workload characteristics of your existing disk subsystem. Using this information, you can load balance your disk subsystem and tune the disk allocation unit size. Finally, because your performance increase can vary according to your computing environment, I'll show you how I tuned and tested my disk subsystem.
Common RAID Characteristics
Most systems administrators add disk subsystem capacity in the form of RAID arrays. When you use NT's internal software RAID technology or hardware-based vendor RAID technology, RAID 0, 1, and 5 are the most common RAID levels for NT Server environments. (For information about RAID level technology, see Table 1 to my August 1998 article.)
When you select RAID configurations, you need to consider many important factors, such as cost and availability level required, in addition to just performance; however, this article focuses on performance. For information about other factors to consider, see the sidebar, "What to Look for When Selecting a Scalable RAID Array."
Regardless of the RAID level you select, use a hardware-based RAID solution if you implement any NT solution that supports more than three disks and requires any level of high availability and performance. Hardware-based RAID adapters and controllers provide much better performance than NT's built-in software-based solution provides and are particularly valuable when you implement RAID 5. With RAID 5, hardware-based RAID adapters offload parity processing and some of the associated interrupt traffic, unlike software-based RAID. This offloading results in improved overall NT performance.
When you're selecting the appropriate RAID level, consider the relative performance characteristics that each RAID level provides. Table 1 illustrates the relative performance level of the various RAID levels in several I/O workload environments and can help you match the appropriate performance to your system's disk I/O characteristics.
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 additional techniques, see "Related Articles in Windows NT Magazine," page 76.
Disk Workload Characteristics
How can you determine what type of disk workload your server is experiencing so that you can appropriately distribute the disk activities across multiple RAID levels or standalone disks? Performance Monitor provides two counters (% Disk Read Time and % Disk Write Time) under the LogicalDisk object that let you identify disk subsystem workload characteristics. The % Disk Read Time counter measures the percentage of elapsed time that the selected disk is busy servicing read requests, and the % Disk Write Time counter measures the percentage of elapsed time that the selected disk is busy servicing write requests.
Using these counters, you can determine how much time your disk spends writing and reading data. These counters provide a high-level view of the type of disk activity you must plan for. Use this information with the information in Table 1 to select the RAID levels that provide the best performance for your environment. For example, if the value for % Disk Read Time is 10 percent and the value for % Disk Write Time is 90 percent, consider either RAID 1, RAID 0, or a standalone disk (for the latter option, remember that you forfeit any disk fault tolerance for improved performance). As Table 1 shows, RAID 5 write performance is lower than other RAID levels and standalone disks because every RAID 5 write incurs four disk operations: read data, read parity data (compare the two using the CPU), write the data, write the parity data. Conversely, if the value for % Disk Read Time is 80 percent and the value for % Disk Write Time is 20 percent, RAID 5 is a good choice.
Many enterprise networks have a mixed read and write environment with some RAID devices experiencing much higher workloads than others. In these instances, you need to load balance your disk devices for optimal performance.
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