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PCIe Throughput for USB3?

post #1 of 15
Thread Starter 
I need a definite answer of how much theoretical throughput comes out of a PCIe 2.0 x1 slot so i can determine how many usb3 ports make sense in using it. Most pcie cards come with 2x USB3.0 but i've seen some with 3 or 4 even, and i'm wondering if it even makes sense. At least if you use 2 drives, on these ports can you maximise each owns performance when moving files from one to another?
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post #2 of 15
500MB/s

PCI-E x2 would be 1GB/s
PCI-E x4 would be 2GB/s

It just multiplies as the x* goes up.

Source
http://en.wikipedia.org/wiki/PCI_Express
post #3 of 15
Quote:
Originally Posted by UsedPaperclip View Post

500MB/s, which is barely more than USB 3.0.

PCI-E x2 would be 1GB/s
PCI-E x4 would be 2GB/s

It just multiplies as the x* goes up.

Source
http://en.wikipedia.org/wiki/PCI_Express

No.

USB 2.0 supports a maximum transfer rate of 480 megaBITS per second. A single PCIe 2.0 x1 slot can support up to 500MegaBYTES per second. Also, with the way USB 2.0 handles duplexing, you'll never really see much more than 240megabits per second which translates into 30 megaBYTES per second.

You can in theory run 15 or so USB 2.0 ports at full speed off of a single PCIe 2.0 slot.

USB 3.0 specification supports 5gigabits per second transfer speeds, though I haven't yet seen a controller that could hit anywhere close to that. Most flash drives and external hard drives with USB 3.0 will max out at about 100 megabytes per second, meaning a single PCIe 2.0 x1 slot can comfortably support 5 USB 3.0 ports, and it would only become an issue if you started doing massive transfers from all five drives at the same time, which would be a very rare occurrence.
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post #4 of 15
Thread Starter 
Ok so it is more than enough for 2 drives, and it can get decent performance for 4 ports as well is what you're saying. Let me run acouple of scenarios by you.

Imagine i have a couple of drives connected at the same time. USB3#1 and USB3#2, and i want to exchange large quantities of files between, them. If both of them perform at 100MB/s, then 1st drive will send the file at 100MB/s and the second will receive it and write it at 100MB/s thus requiring 200MB/s in total, is that correct?

Another scenario, if i want both drives to send files to one another. Drive 1 send Drive 2 Receives, and Drive 2 sends while Drive 1 receives at the same time. Then that will be 200MB/s for tX/Rx times two (x2) so 400MB/s? Am i getting this right or are there issues of what i am imagining to happen?
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post #5 of 15
In scenario number 1, ASSUMING that your USB 3.0 card has good controllers on board, the bottleneck would be the reads and writes of the drives. So your 100 MB/s read would write at 100MB/s and PCIe x1 2.0 would be fine. Again, ASSUMING the USB 3.0 card is good enough.

In your second scenario, then technically, it could require 400MB/s but I think it would still be 200. I should add though, a hard drive, especially an external, cannot efficiently read and write at the same time. Windows would likely pause or halt the writes until the reads were done or vice versa.

Simplified, the drive head cannot be reading and writing at different places at the same time efficiently.
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post #6 of 15
I don't think the number of ports is relevant at all. Remember that USB is a BUS, so all ports share the same bandwidth, unless the card has multiple USB hosts, which is exceedingly unlikely. More ports is just a matter of convenience, like having a hub built into the card.


Technically, PCIe 2.0 1x can't even keep up with 1 USB3.0 host, as 5gigabit is 625mbyte a sec. In practice, its not going to make a lick of difference.

Remember that both PCIe and USB3.0 are full duplex- each can send and receive at the same time, doubling maximum throughput. With this, you'd need hard drives capable of over 400megabyte a second before the bandwidth limits of PCIe to become an issue, even with overhead.

I've read from my USB3.0 external enclosure at over 130 megabyte a second, but really depends on what you are doing with it.
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post #7 of 15
Thread Starter 
I just realized something. If PCIe x1 is less than USB3, it doesnt matter if a card has one 4 or infinite ports, it will always act as a hub, cause even 1 port would be able to reach full usb3 speed, (even if there was a drive that could get that speeds).

So i might just well get a pcie card with 1 or 2 ports and just get a hub if i want more. Cause i was banging my head trying to find a low profile card with 3 or 4 ports, and i now realize its pointless.

BTW i see most of the cards come with some sort of molex. Is the power connection necessary if you need to power usb sticks or 2.5 external drives, or do you need power for the card to work as well? Cause i only plan to connect external 3.5 drives.
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post #8 of 15
Quote:
Originally Posted by therock003 View Post

I just realized something. If PCIe x1 is less than USB3, it doesnt matter if a card has one 4 or infinite ports, it will always act as a hub, cause even 1 port would be able to reach full usb3 speed, (even if there was a drive that could get that speeds).

So i might just well get a pcie card with 1 or 2 ports and just get a hub if i want more. Cause i was banging my head trying to find a low profile card with 3 or 4 ports, and i now realize its pointless.

BTW i see most of the cards come with some sort of molex. Is the power connection necessary if you need to power usb sticks or 2.5 external drives, or do you need power for the card to work as well? Cause i only plan to connect external 3.5 drives.

You can buy that 4 port card if you plan to use four devices and you won't experience any slowdowns in transfers unless you're using external SSD's or something like that.

The molex or sata power connector is there to drive additional power into the ports. USB 3.0 is rated to provide some power and it can only draw 10 watts through the PCIe lanes for that. Therefore, if you have multiple USB Hard drives that consume power, the molex is a good idea. For simple flash drives, there's it's not necessary but still a good idea.
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post #9 of 15
Quote:
Originally Posted by coachmark2 View Post

No.

USB 2.0 supports a maximum transfer rate of 480 megaBITS per second. A single PCIe 2.0 x1 slot can support up to 500MegaBYTES per second. Also, with the way USB 2.0 handles duplexing, you'll never really see much more than 240megabits per second which translates into 30 megaBYTES per second.

You can in theory run 15 or so USB 2.0 ports at full speed off of a single PCIe 2.0 slot.

USB 3.0 specification supports 5gigabits per second transfer speeds, though I haven't yet seen a controller that could hit anywhere close to that. Most flash drives and external hard drives with USB 3.0 will max out at about 100 megabytes per second, meaning a single PCIe 2.0 x1 slot can comfortably support 5 USB 3.0 ports, and it would only become an issue if you started doing massive transfers from all five drives at the same time, which would be a very rare occurrence.
Hey homie.... a Capital B means BYTES, a lower case b means bits, what I said was correct.

Divide Bytes by 8 and you have bits, multiply bits by 8 and you have bytes.
post #10 of 15
Quote:
Originally Posted by therock003 View Post

I just realized something. If PCIe x1 is less than USB3, it doesnt matter if a card has one 4 or infinite ports, it will always act as a hub, cause even 1 port would be able to reach full usb3 speed, (even if there was a drive that could get that speeds).

As I said, it doesn't matter how the freaking thing is connected. The card, along with any USB3-capable motherboard, only has 1 USB3 host! Even on USB3-equipped motherboards, all the usb3 ports will share bandwidth, regardless of number. Its the way USB3 is designed, as well as Firewire and Thunderbolt.
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