How to determine how much I/O load is coming currently with the value of %util appeared by running 'iostat -x' in RHEL ?

Solution Unverified - Updated

Environment

Red Hat Enterprise Linux

Issue

As known, the value of %util appeared by running 'iostat -x' is that the CPU time rate is used to issue I/O requested from device. How can we determine how much I/O load is becoming currently with this value?

Resolution

You cannot determine how much I/O load is present from %util. It only represents the percentage of sample time during which at least one io was outstanding within the scheduler/driver/storage. So with iostat 1 (sample rate of 1 second), a %util of 75% means
there was at least 1 io outstanding to storage 750 milliseconds of the total 1 second sample time. Is it does not represent anything useful in terms of actual load except in the case where you are dealing with a single physical disk.In the case of a single physical disk (such as a direct attached SATA disk), %util represents roughly what percentage of the time the disk was working on an io. With a single physical disk and its single physical disk head, once iostat reaches 100% then no additional io work can be done AT THAT LOAD POINT. Where the load point is comprised of the size of the io, the ratio of reads versus writes, how random or sequential that load is. Determining a load alarm limit is highly
dependent upon both the storage technology being used AND the application. Ultimately the application has more control over
the load point/mixture -- the disk can only process the io, big or small; random or sequential; read or write, as presented
by the application.

So if the question is related to determining some type of quantitative load analysis in relationship to an application load
and how much of an application load is too much then that must be determined via testing with the application and scaling up
the load to measure when the io load response time is no longer acceptable to the application.

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