It would help if you started by showing where your disk would be in the 
device tree. Then I can tell you what (more or less) you need in your 
config file.

USB, or whatever else, is no magic. You can specify explicitly where 
your disk is, and have it show up with a specific device number even 
with other devices attached anywhere.

I seem to remember that way back there was even a tool (in pkgsrc?) 
which extracted your current device setup, and created a config file 
from that, so that you would always get the same enumeration, no matter 
what else showed up on the machine.

The point is, the config file totally, and exactly describes your 
hardware setup. Your suggestion would simply mean that this information 
would be duplicated in the file system.
The config file have the additional "feature" of actually making the 
device appear with the same name, even if you move it around, by just 
changing the config file. Everything else in the system will not have to 
be told after that. And the names exposed, and referred to, are simple 
and short, even though you do have the full tree described in the config 
file.

Someone else mentioned that the problem have grown for the simple reason 
that hardware configurations change much more often now than in the 
past. I would agree with that. However, for vital pieces of the 
hardware, the setup normally don't change that much (such as the disks 
normally used by the system).

So, the device configuration and enumeration is only random so far as 
that if you tell the system that it is okay to give a device a random 
number, it will actually possibly do that.
Otherwise it is totally predictable.

If you, on the other hand, do move your disk around (be that by using 
USB and different ports and hubs, or different controllers), neither the 
old config, nor your new solution will help. The disk will change 
identity (or path) (well, with the old config, it might actually keep 
it's identity, but that's a chancy proposition at best). This is why a 
way to refer to disks by some other property would be nice (such as disk 
labels).

(DEC actually solved this a long time ago, by letting disks have 
identities, which were not in any way related to anything else than the 
disk, and that was the unit numbers on MSCP disks, which you preferrably 
kept unique for the whole system, but as with many other nice things, it 
fell out of fashion with revolution of PC commodity hardware, which have 
a foresight shorter than my nose.)

	Johnny


Masao Uebayashi wrote:
quoted text
>> Have everyone forgotten how to set up their own kernel? Is everyone now  
>> booting GENERIC? (Or just making a copy of GENERIC, with a few patches  
>> without understanding what they are editing?)
>>
>> The whole point being that if you boot a kernel, in which you have  
>> configured the whole system to connect anything anywhere, you should not  
>> be surprised if the device enumeration might seem random.
>> If you want predictable device enumetaion, you can have that, and have  
>> been able to have that for over twenty years...
>>
>> The line
>> wd*     at atabus? drive ? flags 0x0000
>>
>> (to use one example) says that match any wd type disk to any unit number  
>> on any atabus, without doing any closer matching. Ie. kindof 
>> unpredictable.
>>
>> The asterisks and question marks means exactly that. If you want  
>> predictable matching that stays the same at every boot, no matter what  
>> hardware you put on the system, you write explicit lines in the config  
>> instead.
> 
> Imagine if I want to use a USB disk as / on my DELL OptiPlex 745.  The device
> tree of that machine looks like:
> 
> /mainbus0
>   /pci0
>     /puhb0
>       /agp0
>     /ppb0
>       /pci0
> 	/vge0
> 	  /ukphy0
> 	/vga0
> 	  /wsdisplay0
> 	  /drm0
> 	/uhci0
> 	/azalia0
> 	/ppb0
> 	  /pci0
> 	/ppb1
> 	  /pci0
> 	/uhci1
> 	/uhci2
> 	/uhci3
> 	/uhci4
> 	/ppb2
> 	  /pci0
> 	/ichlpcib0
> 	  /isa0
> 	    /lpt0
> 	    /com0
> 	/piixide0
> 	  /atabus0
> 	    /wd0
> 	  /atabus1
> 	    /atapibus0
> 	      /cd0
> 	/ichsmb0
> 	/piixide1
> 	  /atabus0
> 	  /atabus1
> 
> How do you write a kernel config which can always identify my USB disk as
> sd0a, even if I plug random devices?
> 
> Masao
>