"Grumps" said:
For the OP's benefit, I think what you haven't explicitly mentioned is that
SATA150,SATA2,SATA300 (or whatever) refers to the interface (cable) speed,
and not the data rate from this interface and onto the disks themselves.
This is the bottleneck, and is the same for IDE drives too.
So, as Rod says, the difference in speed is down to rpm and sectors/track
(all other things, like good internal design, being equal).
To give some concrete numbers:
1) The media rate of the data coming off the disk is about
60MB/sec at the beginning of the disk. The media rate
near the end of the disk is 40MB/sec. Those numbers might
be typical of a 7200 RPM disk. Storagereview.com keeps a
database of disk performance numbers, with some choices
that go slightly faster.
2) On an old SATA150 drive, the controller design may be
bridged. The first SATA drives that came out were not native.
They used an old ribbon cable design, and slapped a bridge
chip on the controller, to make the transition to SATA. Those
drives might be burst limited to 100MB/sec or 133MB/sec, and
won't do the full 150MB/sec.
3) At this point in time, one would hope a SATA300 drive is
a native design, so no more IDE core + bridge. The design
could still, in principle, be limited by whatever choice
the designer made for the cache memory, or the clock rate
used for the data processing. Only testing will tell how
close it gets to 300MB/sec on a burst.
4) Motherboard chipsets may have internal limitations. Some
Northbridge to Southbridge busses now run at 1GB/sec, so that
won't be a limitation. Previous generation hub busses ran
at 266MB/sec.
5) If a SATA (RAID) controller chip is used on the motherboard,
it has its own properties. If the chip is connected to the
PCI bus, the bus is limited to 110-120MB/sec of a theoretical
max of 133MB/sec. If the chip is connected to PCI Express x1,
the PCI Express can manage 250MB/sec, so wouldn't achieve the
full 300MB/sec. Due to the packetized nature of the data
movement, you won't get the full rate for user data. In RAID 0
stripe mode, it is also possible the combined burst speed of
the disks, will exceed the capabilities of the bus.
To understand what to expect, you have to glance at the architecture
of your computer, and understand where the bottlenecks are. Just
because the drive label says 300MB/sec, you will seldom see that
rate at the hardware level. (You can always be deceived by a
software cache, if the driver uses one. The quality of the
benchmarking software is important, to defeating the software
cache if one is present.)
Paul