tail of two scsis

Jeff Vian jvian10 at charter.net
Tue Jun 15 15:38:06 UTC 2004

Richard Emberson wrote:

> Jose Luis Ricardo Chavez wrote:
>> Richard Emberson wrote:
>>> Jose Luis Ricardo Chavez wrote:
>>>> Richard Emberson wrote:
>>>>> I've got a FC2 system and a scsi disk with /boot and /.
>>>>> In addition, I have two other scsi disks with /home and /usr/local
>>>>> on them (call the disks A and B). Both of these disks
>>>>> have their IDs set to 6.
>>>>> When I boot the system with disk A, disk A can be found and
>>>>> the boot succeeds. When I replace disk A with disk B, disk B
>>>>> can not be found and the boot fails.
>>>>> Other than the possibility that disk B is bad, what else
>>>>> could be the cause?
>>>>> The boot disk is a 7500rpm Quantum.
>>>>> Disk A is a 10000rpm Maxtor.
>>>>> Disk B is a 7500rpm Quantum.
>>>>> Back in my RedHat 9 days, the system used both Quantum disks.
>>>>> Thanks
>>>>> Richard
>>>> Are both disks using the same SCSI id while connected to the same 
>>>> cable
>>>> (SCSI channel)? As far as I remember there is not a "cable select" 
>>>> option
>>>> when using SCSI disks, both disks should use different id's. Put 
>>>> the lowest
>>>> id on the boot disk (A). If the disks are connected to different 
>>>> SCSI channels
>>>> then maybe there is a problem with one of them.
>>>> - Jose Luis
>>> The boot disk is always on the cable. Only one of the disks A and B
>>> are on the cable at one time.
>> Ok, you have three disks, the boot disk is permanently connected and 
>> you connect
>> disk A or B when needed. Is the SCSI BIOS detecting disk B?
> No.
> It detects disk A (disk B not connected) but not disk B (disk A not 
> connected).
> The boot disk in both cases is detected.
Three possibilities that can cause this AFAICT

1)  drive B has failed.

2)  Drive B has the same scsi ID as the boot drive.
If two scsi devices on the bus have the same ID, often neither will be 
seen, or maybe just the first will be displayed by bios but they will 
not function.

3) some sort of problem with termination different with drives A & B.
Drive B is a 7200rpm drive, and may have termination enabled on it. 
 Drive A is 10,000 rpm and likely does not have termination available 
(most of the newer drives do not have termination on the drive).  I 
assume therefore that you have termination on the end of the cable you 
are attaching to.   If drive B has termionation enabled and the cable 
has termination this will cause a problem.

The fact that the boot fails when B is attached leads me to believe that 
option 1 is most likely since a drive that has failed and puts noise on 
the scsi bus can interfere with other scsi devices on that bus.  The 
/home and /usr/local filesystems are not critical to boot so having 
those missing would not be cause for failure to boot. It would appear 
something else is at work besides a drive just not being seen.

Option 2 could also cause B to not be seen by the BIOS, and could 
prevent boot.  You say that B has the same scsi ID as A so this is less 
likely, but still possible if the ID is not what you believe it is. You 
do say that B uised to be used but is now not seen so that leads back to 
option 1 as most likely.

>> - Jose Luis

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