DEfragmentation in IMAC

Hi

We all know that fragmentation of HardDisk decreases File IO operations to great extent.
But that degradation is not seen in IMAC (both Leopard and Tiger)
Can anyone help me with the defragmentation procedure of IMAC?

Thanks,
Tarun

The reason of IMAC having little effect with defragmentation is provided at

[url]http://docs.info.apple.com/article.html?artnum=25668[/url]

Thanks!
I had also need it.