Detecting disk bottlenecks

In most cases, applications can be tuned so that disk I/O does not cause any serious performance problems. However, performance problem may still persist even after application tuning. In this article, we will be addressing common disk bottlenecks and their tuning techniques.

Identifying whether the system has a problem with disk utilization first. Each system provides its own tools to identify disk usage (Windows: Perfmon, and UNIX: sar, vmstat, iostat utilities). For a start, identify whether the paging is an issue (look at disk-scan rates) and assess the overall utilization of your disks (e.g. Disk Transfers/sec on Windows, output from iostat –D on UNIX). It may be that the system has a problem independent of your application (e.g. unbalanced disks), and correcting this problem may resolve it. Read the rest of this entry »

% Disk Time may exceed 100 percent in Controller

When you are running a load test and you may have encounter that the % Disk Time exceeds 100% in Controller or when you are analyzing the graphs in Analysis. Over 100%? Weird right? And when you open perfmon to verify, and it’s still more than 100%. Is it a bug of Windows? What’s the cause behind this? Read the rest of this entry »

Are the disks fast enough?

Physical Disk (instance)\Disk Transfers/sec

To monitor disk activity, we can use this counter. When the measurement goes above 25 disk I/Os per second then you’ve got poor response time for your disk (which may well translate to a potential bottleneck. To further uncover the root cause we use the next mentioned counter. Read the rest of this entry »

% Disk Read Time + % Disk Write Time not equals to % Disk Time?

These article is an extract of “Top Six FAQs on Windows 2000 Disk Performance” by Mark Friedman who also is the author of “Windows 2000 Performance Guide” that explains why the sum of % Disk Read Time and % Disk Write Time is not equal to % Disk Time. Why do we have to explain this? There are times that in Controller and Analysis, your client/user may observed that the total of % Disk Read Time and % Disk Write Time is not 100% and may question this to you. Therefore, a good understanding behind this will give you a build a positive credential to your client/user.

Often when you add the % Disk Read Time and % Disk Write Time counters together, they do not add up to % Disk Time. Furthermore, the % Disk Time counters are capped in the System Monitor at 100 percent because it would be confusing to report disk utilization greater than 100 percent. This occurs because the % Disk Time counters do not actually measure disk utilization. The Explain text that implies that they do represent disk utilization is very misleading. Read the rest of this entry »