9.8. Disk Metrics

A variety of global and per-process disk metrics are available for identifying potential bottlenecks in disk I/O. The tools glance, gpm, and sar all provide methods for accessing some disk metrics.

9.8.1. Global Disk Saturation Metrics

The only global disk saturation metric is the per-disk utilization metric provided by glance, gpm, and sar. Unfortunately, this is an unreliable metric for the following reasons:

  • The metric refers only to the busiest disk.

  • 100% utilization does not mean that more I/Os are not possible. 100% utilization means that each time the disk queue length is inspected, there are one or more I/Os in the queue. There could be one I/O or a hundred. It does not mean that the disk is fully saturated, merely that ...

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