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I'm pretty sure sar's disk utilization % shows how much of the disk is being utilized at any given time, but how does it calculate that? I found:

%util

Percentage of CPU time during which I/O requests were issued to the device (bandwidth utilization for the device). Device saturation occurs when this value is close to 100%.

In the sar man page, but that doesn't answer much for me - the CPU could be issuing I/O requests infinitely but that doesn't necessarily mean that the disk is at capacity - would it not be more appropriate to measure when requests start becoming latent as a ceiling for how hard a disk is being hit?

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sar (a.k.a. sysstat) includes iostat, which reads /proc/diskstats and /proc/partitions

iostat's man page defines %util as:

Percentage of elapsed time during which I/O requests were issued to the device (bandwidth utilization for the device).

Information Sources

  1. sysstat's iostat source
  2. man page

Further Reading

  1. The Linux kernel user’s and administrator’s guide » CPU load
  2. The Linux kernel user’s and administrator’s guide » I/O statistics fields
  3. https://brooker.co.za/blog/2014/07/04/iostat-pct.html
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It depends on which kernel version are you using /proc/partitions or /sys/block/hda/stat or /proc/diskstats, but you answer your question thes file have all the required field to calculate CPU utilisation with sar.

Hope it will help you.

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