Note, though, that "%tt_act" is NOT meaning the disk is taxed this amount of its bandwidth capacity. Especially "100%" does NOT MEAN it is at its limit.

It is like this: the disk (in fact its driver, but nevermind) maintains a queue where commands (like "fetch me some data", etc.) are stored until being executed. In regular intervals the OS queries the disk if this queue is empty or not. The "yes - empty"-answers and "no, not empty"-answers are computed to form a percentage and this percentage is "%tt_act".

While this value is indeed needed to assess the busyness of a disk it is meaningless if it is not combined with other data like averagy queue depth, size of the average read transaction and similar values.

Picture a movie theatres ticket counter: every 5 minutes you ask the clerk if there are people waiting in his queue or not. You do NOT ask if the queue is long or short, if the average customer buys one ticket or several, etc.. From the "yes" (queue bigger than 0) and "no" (queue is exactly 0) answers you compile the "busy" value, but it will not tell you how many people are watching the movie. For this you would need the other mentioned number too.

Back to the disk: if the disk is drowned in many very little requests it might be at 100% but the queue depth will always be very short and in fact a dramatic increase in requests will just make the average queue length a little bigger.

I hope this helps.

bakunin