crshbndct
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Re: USB Sustained Speed?

Wed Sep 21, 2011 4:16 am

I am in the process of shopping around for an external SSD which runs on USB2 (yes such things exist) of around 32gb or so. I know that the SSD would totally choke the USB2 connection, but even with only a 480mbps bus, the read/write/random access speed would be amazing compared to the SD card, and would give performance comparable to a decent 7200rpm HDD. it would also mean that swapping to the drive would probably be fast..
(actually... thats another thought. how fast is the memory on the-r-pi? i know that modern ssds are getting to the point where they are almost as fast as pc66 ram.. and i am guessing that putting a decent ssd on there will be almost similiar to extending the ram by many GB, albeit at a much lower speed. setting swappines up to be very high might turn this into a very fast little computer, but i digress)

One thing I worry about though is the sustained transfer capability and the bus sharing abilities of R-pi. I plan on having a USB hub, with wireless keyboard/mouse/wifi dongle/ssd on it. is the usb port capable of handling all 3 of these things without slowing down hdd access everytime i move the mouse? and if i was, say, copying an 8gb file from the sd to the hdd, would that jam the port up and make input/network stuff lag? using the network port instead of wifi is a definite posibility if this is an issue.
TL;DR: will the usb work with a mouse/keyboard when a sustained transfer of 480mbps is taking place? or will it cause lag?

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Lob0426
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Re: USB Sustained Speed?

Wed Sep 21, 2011 4:54 am

The RasPi has 1 USB 2.0 host from the SoC (the chip), at that 480Mbps. That is going to be divided up among any USB device that is plugged in. The network port on the B model is also part of the USB (The LAN9512 is a USB2.0 hub with Ethernet). If you attach WiFi it will be to the USB also. The fact is that the SD card is faster in certain types of transfers than the USB HDD is going to be. In the Raspberry Pi There are only two storage interfaces available. The SD card and USB.
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jamesh
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Re: USB Sustained Speed?

Wed Sep 21, 2011 8:26 am

Just to add that the SoC memory interface isn't quite market leading (Apple's kit is the best here) but it's damn close.

Moving the mouse shouldn't affect speed overmuch - not much bandwidth required for a mouse.

You rarely (never) get 480Mb/s out of USB2 - lots of overhead to take in to account first.
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Lob0426
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Re: USB Sustained Speed?

Wed Sep 21, 2011 3:09 pm

I think what will slow a RasPi is heavy constant load from Ethernet plus more from a USB HDD. I think there are going to be some trials with some projects that are talking about streaming and simultaneous operations like a FTP server. Might have to rethink some strategies to get it to work smoothly! Keyboard and mice are going to have negligible effect on performance as jamesh stated. I recommend you search the forum as there have been a couple of threads that discuss this topic. You might start with this one.
http://www.raspberrypi.org/for.....#038;t=499
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