[PLUG] Where'd sg0 go?

[Paul Heinlein]

On Tue, 15 Jan 2008, Richard C. Steffens wrote:

> At one point yesterday afternoon the wind was picking up enough to 
> cause some beeping from my battery backup. Since it appeared that it 
> was just getting started and was getting worse, I shut down my 
> various computers. As it turned out, that was the worst of the wind, 
> and we never did lose power out here.
>
> I have a post-it note that reminds me to change the permissions of 
> /dev/sg0 to 666 so I can use my scanner without running as root. 
> When I tried to do that I found that there is no /dev/sg0. I 
> realized that when I shut down the computer yesterday I also shut 
> off the scanner. I didn't power up the scanner this morning. I 
> remember that the instructions for the scanner (when I used to use 
> it on Win98) said that the scanner had to be powered on before 
> booting or the machine would not be able to find it.
>
> Is that true of all SCSI devices? I.e., does the hardware have to be 
> powered up for the system to see before /dev/sg0 will be created?
>
> Is there some way to tell the system to expect a SCSI device 
> sometime in the future without having it powered up before booting?

Some SCSI hardware does great with hotplugging, but it's not 
universally true.

I think SANE ships with a hotplug script that can be used to identify 
recently arrived hardware, but I don't know if it works with SCSI 
devices.

Various Linux distributions include utilities for rescanning a SCSI 
bus. Google for "$DISTRO rescan scsi bus" for your flavor. Under 
RHEL/CentOS, for instance, the directions (which might also work with 
your distribution) were easy to find:

   http://misterd77.blogspot.com/2007_12_01_archive.html

If all else fails, you can "modprobe -r" your SCSI driver (assuming 
your boot device doesn't use it!), power on your scanner, then reload 
the module(s). Your bus will get a clean scan and sg0 should pop up in 
dmesg.

-- 
Paul Heinlein <> heinlein at madboa.com <> http://www.madboa.com/