Large copies to USB 1.0

I've got a laptop that only has USB 1.0. This is great for my USB keyboard and mouse (there are no PS/2 ports at all, so external input devices *have* to be USB, I've never heard about PCMCIA keyboards or mice :-) and also for my USB bluetooth and memory sticks, but it's terrible for a USB hard drive.

Recently I was copying multi gigabyte files to the USB drive and I couldn't stand that it was taking so long and that the laptop would pause and block for tens of seconds. I thought what was happening was that the data was being buffered and then, when the data was actually written down to the hard drive the kernel was so busy handling interrupts (or polling, or whatever) that the computer blocked.

I came up with two workarounds. The first workaround was to set the buffer flushing period to very short, so that the laptop wouldn't block completely for tens of seconds but instead would just get slow every few seconds.

update -3 32 -f 1 -s 1

The second workaround was to limit the bandwidth going down the USB pipe. Rsync is a great tool. This time I couldn't use it to do incremental syncing since in the configuration given checksumming would be far slower than just copying the data file over directly. I didn't want to fill the pipe though since I thought I'd noticed the laptop choking a bit even with the update trick. So what I did was use rsync as a slow cp. Rsync has a --bwlimit option for app level bandwidth limiting which works even within the same computer. So:

rsync -a --bwlimit=384 [srcfile] [dstfile]

limits the amount of data transferred to 384 kilobytes per second. The link can handle up to around 1 megabit per second, but limiting the copy bandwidth keeps the laptop usable and then I just hid the rsync window so that I could continue using the laptop.

rsync --bwlimit wouldn't be enough here. Without the update trick (making buffer flushes shorter and smaller) the buffers would still get filled with hundreds of megabytes of data which would then take around one second per megabyte to go down the wire. With update and no --bwlimit, I was finding slowness and inconvenience. Together though, they made the laptop usable while it was copying large files in the background.

Eventually though, I'll just have to use a laptop that's got USB 2.0 :-).

tiger