Examining and Tuning Disk Performance

Several conditions must exist in order for you to determine that a disk bottleneck exists. These are a sustained rate of disk activity well above your baseline, persistent disk queues longer than two per disk, and the absence of a significant amount of paging. Without this combination of factors, it is unlikely that you have a bottleneck. However, if you suspect a disk-specific performance problem, monitor the following types of counters:

  • Paging counters (under the Memory object): Pages/sec, Page Reads/sec, Page Writes/sec

  • Usage counters: % Disk Time, % Disk Read Time, % Disk Write Time, % Idle Time, Disk Reads/sec, Disk Writes/sec, Disk Transfers/sec

  • Queue-length counters: Avg. Disk Queue Length, Avg. Disk Read Queue Length, Avg. Disk Write Queue Length, Current Disk Queue Length

  • Throughput counters: Disk Bytes/sec, Disk Read Bytes/sec, Disk Write Bytes/sec



Although not reflected in disk activity, the rate of interrupt generation by your disk hardware can have a systemwide performance impact. Disk I/O can sometimes generate a sufficient number of interrupts to slow the performance of the processor. Although this does not constitute a disk bottleneck, it is a processor bottleneck caused by the disk system that can slow the responsiveness of the whole computer. For more information about monitoring disk interrupts and reducing their impact on system performance, see Analyzing Processor Activity in this book.

The following sections describe how you interpret the values of these counters to reveal or rule out a bottleneck.