>> to find how often the head get unloaded, I have completely prevented
>> this effect to occur.
Notice that you have 2 problems:
1- Unloading.
2- Reloading.
>From where I stand, the unloading is normal, harmless, and even
desirable so the real problem is: why is it re-loaded every 3 minutes?
I'd recommend you use /proc/sys/vm/block_dump to try and figure it out.
Stefan