Writing udev rules, short notes

Today I was connecting a second USB harddrive to my NSLU2.

What you get is a few interesting effects, among them device IDs (/dev/sda, sdb...) changing depending on the order in which you attach the drives. Plus, I added a USB hub, which makes the device names change anyway.

The solution for the crazily jumping dev nodes is the udev system, which is part of Linux for quite a while now, but I never really had a need to play with it yet. But the howto is pretty nice and easy to apply.

Still, a few notes:

• The SYSFS{whatever} rules support wildcards, so that you don't have to write the whole descriptor if you don't want to. For example, "Max*" instead of "Maxtor 6" works totally fine. -- That's especially useful since SYSFS{model} descriptors often seem to be filled up with a couple of spaces at the end.
• Some (all?) USB hard drives in fact don't consider themselves to be USB devices. So in this case defining BUS=="usb" will not result in any device links to be created at all. I had to define BUS=="scsi" instead. You should look at the udevinfo output as suggested in the "writing udev rules" howto, it will tell you exactly what setup is necessary for your device/drive configuration.

That's how my two new rules look like (each in one line only): BUS=="scsi", KERNEL=="sd", SYSFS{vendor}=="Maxtor", SYSFS{model}=="L250R0", NAME="%k", SYMLINK="usbhd/maxtor250%n" BUS=="scsi", KERNEL=="sd", SYSFS{vendor}=="Maxtor", SYSFS{model}=="Y080P0", NAME="%k", SYMLINK="usbhd/maxtor80%n"

If you have any questions, let me know.

If not, have fun :)