Native Command Queuing
A simple way to explain how a
hard disk works is: hard disks receive read/write requests from the chipsets I/O
controller which are then buffered on the hard disks memory controller. The buffered requests are then carried out by the hard
disk's on board controller. The hard disk's heads then move to the correct part of the disk (platter)
perform the read or write.
The problem with this method is that the hard disk performs the requests in the order that it gets them.
If the first request is at the start of the disk, the second at the end of the disk and the third at the start of the disk, the hard disk would have to move all the way across to the end of the disk and back again. This process would naturally take longer than having to read three files at the start of the disk. This is why hard disks
manufacturers recommend you defrag your hard drive frequently.
Native Command Queuing allows the hard disk to dynamically change the order in which the read/write requests are done. For the example above the hard disk would reorder the requests so that the first and third requests are done before second. This would save the whole disk head moving across the platter and back again, theoretically halving the time taken to perform the task.
to present NCQ is to compare it to the "Travelling Salesman Problem". NCQ is very similar
to the TSP. Essentially, the TSP is a logistics problem requiring
a travelling salesman who has to visit a variety of different
locations to optimise the route he takes to cover all customers
with the minimum amount of travelling. It is discussed a lot in
logistics courses and there are vast corporate resources devoted
to managing internal versions of this problem - whether relating to
delivery trucks' routes or field service engineers' work plans -
as companies can save a lot of money by working out the optimum route
their vehicles travel. The TSP is a particularly good example to
explain NCQ as it bears this similarity to it: the larger the
number of locations that need to be covered, and the further away
they are, the more there is to
be gained from optimising the route.
Here are two examples of a hard disk travelling to read
data at four different physical locations on a platter. The red
line shows the route the reading head takes. It is obvious from
the images that the second one uses NCQ to minimise the distance
the head travels.
time taken to access a file is the normal seek time the hard disk
displays added to the rotational latency (time taken for all the
spinning). Reducing the latency sufficiently can halve the access
For general hard disk usage such as application
and game loading NCQ doesn't add much benefit. When an application or game is installed on the hard disk all its contents are located around
the same area. So, when an application or game is loaded the amount of
hard disk head movement is naturally
low. Further, the fact that most applications and games load
sequentially causes NCQ to not make a big difference in disk
Running two different applications simultaneously is where NCQ should start to shine. Two
applications at two different locations on a hard disk fighting for access will
benefit from NCQ. With the increase of multi threaded applications and
the use of Hyper Threading technology, NCQ will gain in
technical paper on NCQ (PDF, 670 KB)
The bad news is
that there are a very few motherboards that support NCQ. Our
test setup hardware >>