How do SCSI map to /dev/sd*?
 

does anybody know how SCSI devices on the channels map to the sda sdb etc devices?

I can cat /proc/scsi/scsi to see the devices on the channels and their id and manufacturer but this doens't tell me which one is which block device under /dev.

I would imagine (but don't quote me on this) that the SCSI LUN will map to the sd*. Example:

SCSI LUN 1 == sda
SCSI LUN 4 == sdb
SCSI LUN 9 == sdc

I'd break out the google-monkey for confirmation, but I believe this is how it's done. The SCSI LUN is the Logical Unique Number assigned to each drive, usually with a jumper. Number 7 tends to be the physical card/controller.

no because all the drive are LUN 0 in this machine which has nearly 20 SCSI disks

Typically, the first recognized drive (lowest SCSI id) on the first recognized controller will be sda, next sdb and so on.

is there any command to see the kernel's table of mappings?

this would be much easier and faster as well as knowing that you haven't made a simple mistake...