Bottleneck on PCIExpress?

[pierods]

According to this ad by asus, you need an "expansion bridge chip" between an usb 3 controller or a sata 6 controller and the chipset.

http://event.asus.com/mb/P7P55D-E_U3S6/

Why? PCIExpress has much more than 250 MB/s bandwidth.

If this is true, how come I don't need an "expansion bridge chip" when I plug a pci express x8 RAID controller on my board, that has much more speed than usb 3 for example.

 

[amnotanoobie]

But the usb controller and sata controller get assigned only one PCI-Express lane each.

 

[pierods]

Really?

I didn't know...

But take a RAID controller with SATA disks attached and a pci-e x8 interface, it gets 8 right?

 

[amnotanoobie]

Not really sure, but an x8 card should have access to the full bandwidth of an x8 slot.

 

[roonj]

The current review on Tom's p55 boards explains this limitation of 4 lanes of pci-e and the limit of 2.5.

 

[MRFS]

We're monitoring the transition to SATA/6G storage devices,
and the best explanation we have been able to find to date
came in a block diagram prepared by the good folks at
PC Perspective, founded by Ryan Shrout.

Here's that block diagram:

And, here's the source article:

http://www.pcper.com/article.php?aid=769


Ryan & company were one of the first to illustrate this implementation:
it does NOT provide the maximum bandwidth of the SATA/6G standard
i.e. 6 Gbps / 10 bits per byte (serial protocol) = 600 MB/sec.


Summary, "PCIe x1 Gen2" means x1 PCI-Express lane @ 250 MB/second x 2 (Second Generation)
= 500 MB/second bandwidth ceiling, one direction.


If you want true "6G" bandwidth, check out Intel's RS2BL040 and RS2BL080 controllers:

http://www.intel.com/Products/Server/RAID-controllers/RS2BL040/RS2BL040-overview.htm
http://www.intel.com/Products/Server/RAID-controllers/RS2BL080/RS2BL080-overview.htm

LSI SAS2108 ROC technology, x8 PCI Express Generation 2 host interface and 800 MHz cache enhances the performance of mainstream applications.


Also, I think the ASUS U3S6 add-in card has the same 500 MB/sec. limitation,
because it uses the same chips as found on their P7P55D Premium motherboard:

http://www.newegg.com/Product/Product.aspx?Item=N82E16813131598&Tpk=N82E16813131598


We've asked our Channel supplier to order a U3S6 card for us,
but he recently told us it's not yet available. Lots of photos
of the U3S6 are here:

http://www.maximumpc.com/article/news/exclusive_first_usb_30_and_sata_60_expansion_card_will_sell_30


I hope this helps.


MRFS

 

[MRFS]

> But take a RAID controller with SATA disks attached and a pci-e x8 interface, it gets 8 right?

Not always. Here's why ...

Sometimes, a chipset will only assign x4 PCI-E lanes
to a RAID controller with an x8 edge connector.

It also makes a difference whether the PCI-E slot
-AND- the controller both support PCI-E 2.0.

So, a LOT depends on the pool of available lanes
and the decisions made by the BIOS and chipset
when allocating those lanes to installed PCI-E devices.


RTFM -- Read The Fine Manual (not always "Fine" however

Usually, motherboard User Manuals will provide adequate
documentation describing the number of actual PCI-E lanes
assigned to x16 mechanical slots. I know that ASUS manuals do.


MRFS

 

[MRFS]

ASRock also has an x1 Gen2 add-in card:

http://www.asrock.com/news/events/2009P55/index.asp?c=Overview

http://www.xtremesystems.org/forums/showthread.php?t=232424&highlight=Asrock+P55


The x1 edge connector gives it away:
at Gen2, it's also limited to 500 MB/second.


On this same subject, here's a Press Release from Marvell
which describes their 88SE9128 controller:

http://www.marvell.com/products/storage/6Gbs_SATA_RAID_Controller/release/1335/


MRFS

 

[pierods]

Thanks for the answers.


So, if I understand correctly:

Provided I have

- a board that actually gives 8 lanes to an 8x board,

-and a SATA pci-ex x8 board, that does not use a marvell sata controller, but a special, dedicated controller (think LSI logic)

then my SATA data will travel over 8 pci-ex lanes.

It is not pci-ex dictating that sata controllers must take 1 lane, it's the marvell controller that do.


Right?

 

[MRFS]

If it's SATA/6G functionality you want, yes, you must
shop very carefully because there are not very many
true "6G" controllers available presently.

To future-proof your system, I would strongly recommend
that you consider a reputable manufacturer like Intel:

http://www.intel.com/Products/Server/RAID-controllers/RS2BL040/RS2BL040-overview.htm
http://www.intel.com/Products/Server/RAID-controllers/RS2BL080/RS2BL080-overview.htm

Or, if you are looking at any other models, make a point
of asking all your questions of the manufacturer's
Sales and/or Tech Support, BEFORE deciding.

The entire IT industry is going through another "growing pain"
as we ramp up to USB 3.0 and SATA/6G and SAS/6G peripherals.

The whole situation could change rapidly in the next 2-3 months
particularly as SATA/6G SSDs become available (an event many of us
are awaiting anxiously). None of this matters quite as much for
rotating platters, because their actual data rate is dictated by
the rate at which binary digits pass under the armature's
read/write heads.

If you already have a motherboard with an available x8 or x16 PCI-E slot
and its chipset assigns a full x8 PCI-E lanes to that slot, it should accept
a RAID controller like the ones from Intel above.

I believe that LSI builds Intel's controllers, so look at LSI's offerings too.

You're even better off if your PCI-E slot is "Gen2" i.e. PCI-E 2.0
because it doubled the bandwidth for each lane to 500 MB/second
in each direction. PCI-E 1.0 is 250 MB/second in each direction.


MRFS

 

[MRFS]

http://www.highpoint-tech.com/

The Highpoint Rocket 620 and 622 are also "PCI-E 2.0 x1":
just look at the edge connector.

Again, all such controllers have a bandwidth limit of 500 MB/second
in each direction, whereas "6G" means 6 Gbps / 10 = 600 MB/second.

Thus, any "PCI-E 2.0 x1" controller is not "true 6G" for that reason:
it just can't be if it only uses a single PCI-Express 2.0 lane!

The logic on the card may, in fact, communicate with the storage device
at 600 MB/second, but that card turns around and slows down whenever
it communicates data to and from the motherboard's chipset.

So, the real "bottleneck" is the x1 PCI-E 2.0 lane.


MRFS

 

[pierods]

Got it, thanks!