Where do you connect multiple SATA devices?

My mobo (ASUS P8P67) has:
4 Intel P67 SATA3
2 Intel P67 SATA6
2 Marvel SATA6

So the question is, if I have a SATA6 HD, in addition to some SATA3 devices, should I connect the SATA6 HD to the Marvel controller, or the Intel Controller?

Does "spliting up the load" make a difference here?

No, the Marvell are NOT as fast as the native Intel SATA3 ports, and no mechanical HDD benefits from the SATA3 ports. Don't confuse the 'SATA 6.0 Gb/s' with the speed; any HDD is 120~140 MB/s and at best 1/2 the SATA2 speeds of ~300 MB/s.

The benches I've seen the Intel SATA2 with mechanical HDD {SATA2 or SATA3 'Interface'} is slightly faster Writes and negligible Read differences.

HDD SATA2/3, SSD SATA2 - 4 Intel P67 SATA2
SSD SATA3 - 2 Intel P67 SATA3
When you run out of other SATA ports - 2 Marvel SATA3

Real world limits; varies on MOBOs:
Marvell SATA3 9120/91XX {x1 lane} 360~395 MB/s ; P8P67
Intel SATA2 285~295 MB/s
Intel SATA3 515~550+ MB/s

Marvell SATA3 9182 {x2 lanes} 515~550+ MB/s ; Maximus IV Extreme

Yep pretty much you've hit the nail on the head, your HDD probably has a larger cache than the SATA2 counterparts.

Try it your self with both your SATA ports:
ATTO -> http://www.attotech.com/products/p [...] _Benchmark ; this is the authors site.
HD Tune -> http://www.hdtune.com/download.html ; this is the authors site.


Older post {Dedicated $ SATA3 Card with onboard R/W Cache in other words as good as it'll ever get for SATA3}:

WD5002AALX http://www.harddrivebenchmark.net/ [...] WD5002AALX

For perspective:
Shopping a HDD the ideal choice is the cheapest with best warranty. HDD speed differences are like splitting hairs. The best thing you can do to improve performance is use AHCI and set auto defrag.

Performance measurement is all about IOPS and comparative latency - 12ms vs 0.1ms or 120X slower HDD or 1200% faster SSD depending upon how you look at it, and minimum 80X+ slower HDD or 800% faster SSD in IOPS.
HDD ~75-100 IOPS
SSD Vanilla 8,000 IOPS to OCZ Vertex 3 ~60,000 IOPS