Re: SAS scsibus target numbering

[Eduardo Horvath <eeh%NetBSD.org@localhost>]

On Thu, 26 Jul 2012, Edgar Fuß wrote:

> I have a (mpt) SAS with seven discs connected.
> The discs attach as sd0..sd6, but the SCSI target numbers are 0..5 and 7.
> It appears to me that someone is skippig ID 6 for the controller.
> It doesn't hurt too much, but it took me a while to find out why detaching 
> targets 2, 3, 4 and 5 worked and 6 didn't (of course, 7 worked).
> 
> Is there a reason for this behaviour?

"target IDs" are assigned by the mpt firmware.  You can change them 
arbitrarily by messing with mpt either from BIOS or their command line 
utilities.  The idea is that they can emulate a SCSI bus and thus boot 
Windows.

ISTR that the mpt interface is the same across their SAS and parallel SCSI 
adapters.  With parallel SCSI, the HBA driver allocates itsel a SCSI ID, 
usually 6.  This is meaningless for SAS.  

Some versions of the mpt firmware also support RAID.  When a RAID volume 
is created by the firmware it is also allocated a target ID.   

The whole thing was a gigantic mess.  When I was at Sun we switched from 
using the "target ID" to identify the drives to using a combination of PHY 
numbers for directly attaced devices and GUIDs for RAID volumes and 
devices on the other end of SAS switches.

I can't say I know anything about the NetBSD mpt driver, but I figure if 
you don't like the way the "target IDs" are assigned, go into the firmware 
and swap them around until you're happy.

Eduardo