GA-P55A-UD6 PCIe Setting

I have a Crucial RealSSD C300 that I would like to take advantage of the Sata3 interface (And USB 3.0), but if i am not mistaking when I set that PCIe controller to Gen2 to enable the Sata3 and USB 3.0 speeds it sets my PCIEX _16 slot down to 8x. I also have an MSI Cyclone 1 GB GTX460. How much performance hit will I take running at 8x on my PCIe slot? Would it be better to run the gcard at 16x and the SSD at the Sata2 speeds? I have read that Sata2 is a bottleneck for SSD's, I am not sure on the graphics side though.

Any help or insight would be appreciated.

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  1. Hi there,

    Your GA-P55A-UD6 is a nice motherboard. So is the Crucial RealSSD drive.

    The PCI-E 2.0 serial slot is x 16 for your graphics card, but when you use two PCI-E slots, each of them each run at 8 channels. The PCI-E 2.0 standard transfers 500 MB/s per channel, so the transfer rate for each PCI-E x8 would be 4000 MB/s.

    Your SDD has a SATA 3.0 interface which is spec'd at 600 MB/s, (6 Gb/s= 480 MB/s) so there shouldn't be a problem.

    However, your MB has 2 SATA 3.0 ports using the Marvel 9128 chip, so why not consider using one port (SATA_6) for your SDD, and setting it as the active drive in your BIOS? Use the PCI-E x 16 slot for you graphics card.

    Not sure where the USB notation mentioned comes into play, since the Crucial RealSSD has a SATA 3.0 interface.
    What size is your Crucial Real SSD?
  2. The specs say that the Marvel chip and the NEC (USB 3.0)supplies the 16 PCIe lanes, when you set either for Gen2 (which enables Sata3 or USB respectively) the sata3 or USB takes up so many lanes of bandwidth thus reducing the PCIe slot to 8x. I have the 256GB model.

  3. ???

    Look at the system block diagram on page eight of your manual; both the Marvell SATA3 controller, and the NEC USB3 controller use the PCIe lanes available from the P55 platform control hub; the two graphics slots use the PCIe available directly from the CPU, and are thus independent of the settings for the NEC/Marvell... The only thing affecting whether you get eight or sixteen lanes on your graphics is whether only one, or both slots are in use - if one slot remains empty, the other is x16; if anything (even an x1 card) is plugged into the second, both will bifurcate down to x8...

    In addition, the 460 is likely at the 'hairy edge' of being able to saturate eight PCIe lanes - if you actually see any performance decrease from an x8 slot, it will be infinitesimal...
  4. Hi there,

    I agree with Bill, the Marvel 9128 chip uses 2 PCI lanes from the P55 platform controller hub, not the PCI-E lanes that are directly connected to the CPU. So using the SATA 3 port won't effect the PCI-E speed of your 16 x Video card.

    Either way you are going to have a really nice high end system with your new components & 256GB SSD.
    Great choices - Congrats!
  5. Not discounting anything you are saying but I wonder if that is a misprint? From everything I have read using Sata 3 port or USB 3.0 limits your PCIe bandwidth to 8x on the 16x lane. Even if only 1 card is used. I realize that running any dual set up with the P55 chipset and you are limited to 8x on both lanes. On page 54 of the manual is what I am going off of? And it says "Please note that when only one graphics card is installed on the PCIEX16 slot, it will operate at up to x8 mode if either one of the two controllers is set to PCIe Gen 2" Am I still not getting it? I am excited for this build. It is only my second build for me, although I am a Network Administrator by trade, I never get time "play" with my own computers. I am currently on an 32bit XP machine with an E6300 and a 8800GT with 3 gigs of ram. So migrating to a Win7 64bit with an I5-750 and the gtx460 with 8gigs of ram will be nice :wahoo:

  6. Ahhh - I'm confused as well! I see the manual entry (thank you for pointing it out by page, by the way...), but I can't fathom what it means. I have no recall of anything relevant in the PCH spec sheet, but the damned thing is 892 pages long, so very little of any significance has 'stuck' [:bilbat:6] - I'll need to do some serious digging. As the DMI link to the hub is the only way the CPU 'sees' the hub itself, I can't fathom how the PCH config could possibly affect the on-CPU PCIe lanes, and the 8 hub PCIe root ports are, by design, and in hardware, gen 2.0
  7. Best answer
    Hi Erik,

    I downloaded your manual, and it is confusing in regard to the PCI-E channels.

    Take a look at page 8 at the block diagram. The CPU has 16 PCI-E 2.0 channels (500 MB/s) , and the Graphics PCI-E slots PCIE16_1 and PCIE8_1 come from the CPU. The switch chip lets you use 1 PCI-E x16 slot, or 2 PCI-E x8 slots. That uses up the 16 channels.

    The CPU is connected to the PCH (Platform controller Hub ) by the DMI interface, and the P55A PCH has its own 8 PCI-E channels. These channels are only1.0 (Gen 1) (250 MB/s)

    On the right side of the diagram there is a PCI-E x4 bus. That controls the 1 PCI-E x4 slot, or 3 PCI-E x1 & 1 JM362 controller of 2 additional SATA 3 ports.

    On the left side of that diagram you see 1 PCI-E x4 Bus, 1 channel for each of the two RTL8111D Ethernet ports, and 1 channel for the Marvell 9128 controller (2 SATA 6 ports), & 1 channel for the NEC controller (2 USB 3 ports). Your board has an Integrated Peripheral BIOS setting to run the Marvel 9128 and NEC D720200F1 controllers at either "Gen 1" 250 MB/s or "Gen 2" 500 MB/s speed.

    Here's the rub.

    To get full advantage of the SATA 6 (600 MB/s ) and the USB 3 (4800 Mb/s = 600 MB/s) ports, it requires using the "Gen 2 (500 MB/s) setting, or the bandwidth at "Gen 1" would be 250 MB/s, a bottleneck for the faster SATA 6 and USB 3 devices. Unfortunately they did not use a PEX 8608 bridge chip on this MB to control the data flow speed.

    So evidently the board is engineered to cut the Graphics Card slot to x8 when choosing "Gen 2" for these other integrated peripherals.

    You are correct in making that point, good pickUp!!

    So the PCI-E 16X card will run at 8X, if you connect your SATA 6 SDD drive either way.
    My choice would be to run the SSD at "Gen 2" 500 MB/s to get the fastest I/O speed possible, and accept the slower x8 Graphics card which you may not notice.

    John VK
  8. Thats what I suspected. I will do just that. From what I have gathered my video card will really not be bottlenecked by the x8 bandwidth anyway. Thanks for your help. I plan on documenting my build in the new builds forum just for kicks and grins. The new pc:

    Antec p183
    MSI 1GB Cyclone GTX460
    Seasonic x750 Gold
    8 GB Gskill Ripjaws DDR3 1600 Cas9
    Thermalwright Mux120 cooler
    Handful of Nexus real quiet case fans
  9. Best answer selected by onefastford.
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