I know what the disk io, disk queue length and other measures are, but what does 'Highest active time' mean?
Is it the amount of time it is busy handling requests, or something else?
When it is high, does it mean the CPU is busy doing some IO work, or is it just indicating that the disk is busy handling requests?
I'm trying to work out if 50% active time means that 50% of the time the disk is either seeking, reading or writing, rather than the kernel is spending 50% of it's time servicing IO requests.
Edit
Another quick data point here. If you look at the difference between an SSD and a physical disk, the SSD has significantly less activity, so I guess this really means the amount of time the operating system is waiting for the disk to respond and returning data.
Best Answer
I’m pretty sure it is the same as the Perfmon counter "PhysicalDisk _Total %time". (It tracks this counter in lock-step.)
Perfmon gives this description of that counter:
% Disk Time is the percentage of elapsed time that the selected disk drive was busy servicing read or write requests.
Basically, it is the percentage of elapsed time that the physical drive(s) were busy. So Even though your MB/s usage may be low, the % busy time can be high if the disk is heavily fragmented, or is otherwise thrashing on lots of small operations.
On a server where you would try to match the right level of high-performing disks to the workload, you would consider the disk subsystem to be a potential bottleneck if the % active (busy) time plateau-ed above 80%. On a workstation if this number is high, it just tells you that your disk performance is maybe what is slowing down your work.
As to your last question: I'm quite certain it is the physical disk and not any kernel or process time. While waiting on the physical diskthe kernel blocks whatever processes is waiting on that disk operation, and uses the CPU for whatever else needs doing.