| Seagate Barracuda 7200.9 500GB |
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Here is how NVIDIA puts NCQ in their nForce4 chipset technical brief...
Unlike purely electronic pieces of a computer, hard drives have a mechanical element and can suffer from the laws of physics. This characteristic limits how fast a hard drive can access media and retrieve data. Mechanical limitations can only be optimized to a certain degree, but the efficiency of the internal workflow can be dramatically increased by intelligently managing the workflow . For example, workflow can be managed by using Native Command Queuing (NCQ), a command protocol in Serial ATA that allows multiple commands to be simultaneously outstanding within a drive (Figure 3).
The best example of workflow management is the expedient transportation of elevator passengers. Imagine four people entering an elevator and successively pushing a button for one of the following floors: 4, 2, 6, and 3. In a non-NCQ environment, the elevator stops at the fourth floor (bypassing the second and third floors), goes down to the second floor, goes up to the sixth floor (again bypassing the third floor), and finally descends and stops at the third floor. This is not an efficient way of transporting passengers. In an NCQ environment, however, the elevator stops at the second, third, fourth, and sixth floors, in that order. And for hard drives, NCQ can help overcome the drive’s mechanical limitations and increase storage performance on random workloads by allowing the drive to internally optimize the order of the commands.
And here is how Seagate describes NCQ...
NCQ is a command protocol in Serial ATA that can only be implemented on native Serial ATA hard drives like Seagate's. It allows multiple commands to be outstanding within a drive at the same time. Drives that support NCQ have an internal queue where outstanding commands can be dynamically rescheduled or re-ordered, along with the necessary tracking mechanisms for outstanding and completed portions of the workload. NCQ also has a mechanism that allows the host to issue additional commands to the drive while the drive is seeking for data for another command.
NCQ allows the drive to set up the direct memory access (DMA) operation for a data transfer without host software intervention. This is also called first party DMA - it means that the device is capable of complex sequences of operations without CPU intervention. The drive itself knows the current angular and rotational position of the drive head. The drive then selects the next data transfer to minimize both seek and rotational latencies.
If you didn't quite understand all of that, basically, Native Command Queuing tries to re-order I/O requests on the hard drive. Rather than executing I/O requests in the order they are received, NCQ helps gather multiple I/O requests and executes them based on their proximity to the hard drive head's position. Therefor, in a scenario like the one shown in the graphic above it would take a drive without NCQ two rotations to complete the four I/O requests in order. In a NCQ capable setup, it would re-order the four I/O requests and save time by only making one rotation. Sounds promising doesn't it? Let's now take a look at each drive individually.