Investigating Disk Usage by Applications

Applications rarely read or write directly to disk. Instead, application code and data is typically mapped into the file system cache and copied from there into the working set of the application. When the application creates or changes data, the data is mapped into the cache and is then written back to the disk in batches. The disk is used only when an application requests a single write-through to disk or it instructs the file system not to use the cache at all for a file, usually because it is doing its own buffering. For this reason, tracking the cache and memory counters provides a way of investigating disk usage by your application. You can find information about monitoring cache and memory counters in "Evaluating Memory and Cache Usage" earlier in this book.

When monitoring disk usage by applications, you might find that applications that submit all I/O requests simultaneously tend to produce exaggerated values for the % Disk Time, % Disk Read Time, % Disk Write Time, and Avg. Disk sec/Transfer counters. Although throughput might be the same for applications that submit I/O requests intermittently, the values of counters that time requests will be much lower. It is important to understand your applications and factor their I/O methods into your analysis.

If you are writing your own tools to test disk performance, you might want to include the FILE_FLAG_NO_BUFFERING parameter in the open call for your test files. This instructs the Virtual Memory Manager to bypass the cache and go directly to disk.

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