USB2 doesn't keep up with capture under 2.4.27
 

Have a 4-disk RAID 5 array of 250Gig HDs that I am looking to use for performing video capture and editing. Two of the disks are internal, and two are external and connected using a USB2 card. Running under a Debian installation with a 2.4.27 kernel, I can capture for a few minutes, but then I start to drop frames, and the percentage dropped gets worse as time goes on.

Would going to a 2.6 kernel where I gather there are some additional features available "fix" this, or do I need to look at moving to firewire or something else? I'm reticent to move to firewire as I was having troubles with my system "forgetting" that it could talk to some of the disks after a reboot using firewire enclosures, but if I have to then I guess I have to :-)

Thanks,
Ewan

My understanding is that although USB specs apear to have more bandwidth, it is less efficent, has more overhead, or requires more processor time than firewire. Firewire was originaly designed for PAL and NTSC video transfer. Its speed matches up exactly with the video requirements. So if USB is slower than id meagine it just couldnt quite keep up. Dont quote me on this, wait to see what someone else says.

Would adding a second USB2 card help (so there's only one disk per card), or is the CPU the limiting factor?

Im not sure, I'd emagine it depends on the card. The throughput is shared by all the ports on one controller. Just like if you use a USB hub the maximum throughput is limited by the single connection to your computer.

ie: if your card has only one controller, then using a seperate card may help.

I'm just guessing. There are loads of 'FIREWIRE vs USB' articles if you search in google. Just finding the right one is a problem : P

I came across review comparing a USB and Firewire external disk. USB managed better on data transfer from the disk, while FIREWIRE worked a bit better the other way. If its that subjective then maybe only FIREWIRE is only adequite for video transfer when connected directly to a camera as thats its origional primary intended purpose. But then your using raid so their should be space.

If your using sarge or some other testing it may just be a bug

disclamer: I am guessing, need to get someone elses opinion.

Im just wondering, are you using software RAID? it lets you use USB and other drives in an array?

Yes, I used mdadm 1.7 to create the array. Idea was that I would get somewhat better throughput on a RAID array than writing to a particular disk. Not to mention the "convenience" of not having to move large files around to keep the capture disk clean.

Well, I got my answer by using a second card and moving to kernel 2.6.7 - sort of.

Put in the second card, and it showed up in usbview, but the disk attached to it showed up as being attached to an "unknown" usb hub at only 1.1 speed.

Figured this might be a 2.4 kernel issue (with USB), and so rebooted into kernel 2.6.7. Nope, the second card still showed up "funny". However I had somehow convinced it to load ehci-hcd (had not been able to do that before) and so the first disk was running at Hi Speed.

Just for grins I pulled the second drive off the second card, and plugged it back into the first card. Now both disks were showing hi speed.

Having read that the USB drivers for kernel 2.6 were more "efficient", I decided to see what would happen. Sure enough, I can now capture (at least 10 minutes) without a single dropped frame.

So, adding a second card evidently did "fix" the problem, just not directly :-)

FWIW,
Ewan