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Misunderstanding of judging disk limit performance: determine whether the disk reaches the bandwidth or iops limit only by the% util metric in iostat

Background:

When judging whether a disk has reached its limit performance, someone always uses the% util metric in iostat-x to confirm whether the disk is a bandwidth or IOPS bottleneck. In fact, this is not correct. Please note as follows:

Conclusion:

The% util in iostat is basically useless, and svctm has no reference significance.

Whether the disk reaches the real limit bottleneck, you need to refer to the limit bandwidth and IOPS measured by tools such as fio.

% util and hard disk device saturation

% util indicates that the device has a time ratio of Ihambo (that is, not idle), regardless of the number of Icano, only whether it is available or not.

Because modern hard disk devices have the ability to process multiple Imax O requests in parallel, even if% util reaches 100%, it does not mean the device is saturated.

To take a simplified example: it takes 0.1s for a hard disk to process a single IZP O, and it is capable of processing 10 IZP O requests at the same time. When the 10 IZA O requests are submitted sequentially, it takes 1 second to complete, and the% util reaches 100% in a sampling period of 1 second; while if the 10 Icando requests are submitted at once, all of them are completed in 0.1s, and the% util is only 10% in the sampling period of 1 second. It can be seen that even if the% util is as high as 100%, the hard drive may still have the capacity to handle more Icano requests, that is, it is not saturated.

So is there any indicator in iostat (1) that can measure the saturation of hard disk devices? Unfortunately, no.

Reference article:

[1] IOSTAT: http://linuxperf.com/?p=156 that can be misread easily

[2] Deep understanding of iostat: https://bean-li.github.io/dive-into-iostat/

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