Whoa, it went from 150MB/sec to 3000MB/sec? Or should that be 300?
It's 300, not that it matters. The actual transfer speed is around
60MB/second. The advantage of SATA-2 is the inclusion of Native Command
Queuing (NCQ) not the faster bus speed. NCQ allows the disk to handle
multiple requests at the same time, and more importantly reorder those
requests to minimize head movement. Imagine a situation where you have
three files, two on the outer edge and one on the inner edge of the
platter, call those file A,B and C. Suppose the OS requests those files in
the order A, C, B. In a PATA or SATA I drive the heads would have to move
to the outer edge, then to the inner edge and then back to the outer edge.
Potentially having to traverse the entire width of the platter three
times. In an SATA II drive (or a SCSI drive) the controller would look at
the position of the head, move it to the closest file and then move to the
next nearest file and so forth. If the head were sitting in the middle of
the platter when the first request was received the total head movement
would be only 1.5 crossings of the platter as opposed to three. If the
head were already on one edge then it would only have to cross the platter
once. So the head movement is reduced by a factor of 2 to 3 in this
example. In a file server with lots of disk accesses this is a huge
advantage. In a desktop you are unlikely to see any difference at all
because desktop systems usually only fetch one file at a time and most of
the time they aren't doing anything at all.
