What if the parameter for --size option is smaller than the target device?

Hi,

I am writing a test with fio tool (2.0.8) and I am facing a problem.

I just want to submit 512B random write to a block device.
In my case, the size of the device is 2MB and the amount of writes (specified by --size option) is 32MB.

like this (A line from device-mapper-test-suite. Is my command wrong?):

      ProcessControl.run("fio --name=test --filename=#{dev.path} --rw=randwrite --ioengine=libaio --direct=1 --size=#{@param[0]}m --bs=512")

The problem is, the runtime is too short.
The block device is just a single HDD so, it will be 30sec or so. But it's actually 3sec...

My guess is that fio uses min(device size, param to --size) as the amount of writes here. To investigate it, I increased the size of the device to x10 and saw the runtime gets x10.

My questions are:
1. Is my guess correct?
2. What shall I do to make fio work as I want it to do?

Thanks,
- Akira