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Asked: 2019-04-18 13:56:17 +0800 CST

How does sar collect/calculate disk utilization %?

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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?

linux

2 Answers

  1. 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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  2. 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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