The disk I/O is measured in time spent servicing requests. The way we measure disk I/O on Linux is “wall-clock time with read or write requests being processed by the disk”. This is obtained from a counter the OS keeps in /proc which is read by the Server Monitor Agent. Disk I/O is displaying what percentage of time a disk is in use by a read or write command. For example, if for 55 seconds out of the minute a disk is reporting a read or write, then the value shown will be 92% Disk I/O Utilization.
This only shows a percentage of time that the disk is being used, it does not reflect a fullness or amount that is being read or written. Most commonly Databases show very high Disk I/O utilization due to the large number of read write requests. This is not tracking a metric equivalent to either IOPS (I/O Operations per second), or to “capacity” - how close to maximum possible your usage is. So you can imagine a situation where a program logs twice a second bringing usage to 100%, even though you could also be successfully writing 10x that much data without issue to the same disk.
We gather this information directly from the Linux OS. by reading the
/proc file. From this we gather
IO Utilization (percentage of time spent reading and writing)
IO Rate (amount of data being read written over time in kb/s
IO Operations per second ( how many operations are executed per second)
Disk Space utilization