long standing aggravation

[John Summerfield]

Clyde E. Kunkel wrote:
> On 10/22/2009 02:45 PM, Gene Czarcinski wrote:
>> <snip>.
>>
>> So, if /proc/scsi/scsi can be created correctly, why can't anaconda do 
>> it?
>>
> Looks like anaconda is using the BIOS boot disk order, not the bus 
> order.  At least it does that on my ASUS P5K-E mobo and P5E mobo and 
> some older ASUS mobos that I used to have.
> 
The "BIOS boot order" is not as easy to find as you'd like:
http://www.gnu.org/software/grub/manual/html_node/Device-map.html#Device-map
"The reason why the grub shell gives you the device map file is that it 
cannot guess the map between BIOS drives and OS devices correctly in 
some environments. For example, if you exchange the boot sequence 
between IDE and SCSI in your BIOS, it gets the order wrong."

A long-existing problem with SCSI disks is figuring the order, reliably. 
SCSI controllers can support seven or fifteen devices, and one can 
add/remove devices (on some at least) at any time, including while the 
system is up and running. I don't recall whether the host OS could tell 
which connector any particular disk was attached to, but I suspect not.

I could never understand how calling all disks "scsi" would help.

I can imagine that polling a SCSI adaptor, checking what is attached to 
each connector might be time-consuming (especially if it relies on a 
timeout to detect "nothing here, move along") and might miss something 
that is slow to respond - perhaps getting different results between 
cold-stand and warm-start, while relying on interrupts as devices become 
ready could result in random ordering, even on the same hardware and 
potentially between reboots.

No doubt the use of GUIDs solves the problem of recognising individual 
disks, unfriendly to humans though they be.

I have wondered whether /dev/sda might be an internal disk one boot, a 
USB disk another even when both are present both times. I think I could 
build a kernel so they do change, by reordering the loading of relevant 
drivers.



-- 

Cheers
John

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