User`s guide

Chapter 4. Implementing the SAN Data Gateway 295
The SAN Data Gateway must add this extra layer of addressing as the host is no
longer directly attached to the disk devices but will see a single target ID, the
SAN Data Gateway. In a regular SCSI environment with the host attached
directly to the SCSI device, the host is able to recognize the target and LUN ID of
that device. Since we add the SAN Data Gateway in between the host and the
device, the host is no longer directly attached to the SCSI device to recognize its
target or LUN ID.
4.4.3 Assigning LUN IDs
The Gateway assigns LUN IDs to all the attached devices, up to the maximum of
255, and it creates a map of the actual disk device target and LUN ID to its own
assigned LUN ID. This map is stored in nonvolatile memory within the Gateway.
4.4.4 Adding SCSI devices
New SCSI devices can be added at any time. If a new device is added, for
example, another volume in the IBM Enterprise Storage Server has been
assigned to a SCSI port on a San Data Gateway, the SCSI buses must be
re-scanned to detect the new device. This can be done from the service terminal
using the scsiRescan command or from the StorWatch SAN Data Gateway
Specialist. If using the Specialist, select and highlight the SCSI bus that the
device has been assigned to, and select Controls-> SCSI Channel-> Re-Scan
SCSI Bus. As it rescans, the SAN Data Gateway will assign the next available
LUN address to the new device. Refresh the data on your Specialist window by
selecting View-> Refresh SAN Data Gateway.
Once a device has been assigned a LUN ID by the Gateway, it will maintain that
ID since it was written into the device map. This is useful in case the device is
lost or needs to be replaced. Remove the old device, set the new device to the
same SCSI bus target and LUN as the old device and attach it to the same
channel. You must rescan the SAN Data Gateway SCSI bus for it to update its
data. You will notice that the replaced device has kept the same assigned LUN.
If a device is no longer attached or no longer required, then the assigned LUN is
also no longer required. To free up this assigned LUN ID, you can issue the
mapWinnowDatabase command from the service terminal. The existing devices
will maintain their assigned LUN IDs.
If a complete rebuild of the SCSI device map is desired or required, this can be
done from the service terminal only. You would issue the mapRebuildDatabase
command. This command deletes the existing device map and the SAN Data
Gateway will restart. When it is rebooted, a new scan of the SCSI buses is done
as if the system was brand new, and LUN IDs will be assigned as described
earlier.