How does defrag use low-priority disk I/O and CPU?
Defrag always uses low priority CPU and disk I/O except when defragging the MFT (to prevent long blocking periods on the MFT). The type of defrag you choose (scheduled or manual) makes no difference. Using low priority CPU and disk I/O does not prevent Defrag from using more system resources when you are not using the computer, such as when the scheduled defrag runs overnight. You can see this by running the command-line version of Defrag with just your Task Manager running. Defrag will eventually take a lot of resources, but if you start doing something else on the computer, Defrag will back off. This means that the defrag process will be slower when you use the computer, but then again, you won’t notice the same hit on performance as you did using the Windows XP Defragmenter.
